The Sudoku craze is getting a little out of hand.

British Airways recently issued a memo to its 13,000 cabin crew members reminding them that working on games like the popular Japanese number puzzle during takeoff and landing is prohibited for safety reasons. BA said the move was prompted by passenger concerns that crew members were doing puzzles.

But the movement continues to grow, and there is a mini-industry springing up to sell sudoku in a variety of new forms. A number of software makers are introducing versions for cellphones and personal digital assistants, including popular models made for Palm Inc. Retailers such as Discovery Channel Store are introducing hand-held electronic sudoku games, and board-game makers like Millburn, N.J., based Briarpatch Inc. are coming up with their own versions.

Web sites such as sudoku.com are offering premium services where players order an unlimited number of puzzles to play online for around $15. Other sites are also launching new twists, such as a two-player version from number-logic.com that allows players to compete head to head.

Sudoku is even showing up in math classes, thanks to a growing number of Web sites that make it easy for teachers to download age-appropriate versions of the puzzle.

The companies are betting that the new formats can continue to stoke sudoku's popularity in the U.S. and keep it from going the way of previous game crazes like Rubik's cubes, Tetris, and electronic solitaire. Although sudoku has long been worked on paper, as a game of mathematical logic, it is perfectly suited for a migration to digital devices, which can easily be programmed with simple formulas that keep generating puzzles. Also, it is easy to play on a cellphone touch pad and can played anywhere in the world without translation.

There are hints of a backlash, as some hard-core puzzle solvers are trying to cut back. Auren Hoffman, a 31-year-old entrepreneur from San Francisco, took two months off from sudoku after deciding that his obsession was hurting his productivity. Now he is playing again -- but just one puzzle a week.

After first catching on in Japan in the 1980s (its name is a Japanese word commonly translated as "only single numbers allowed"), sudoku quickly hopscotched across the globe. It was introduced in England a little more than a year ago. The New York Post -- owned by News Corp., whose holdings include several British newspapers -- brought the puzzle across the Atlantic last spring. More than a hundred U.S. newspapers now carry the puzzle and sudoku puzzle books are popping up on best-seller lists. The first sudoku World Championship will be held in March in Italy.

Hudson News Co.'s Hudson Group, based in East Rutherford, N.J., says that the puzzle's popularity has managers at its Hudson News stores -- which are in more than 50 North American airports -- rushing to restock pencils. They're also keeping sharpeners on hand for customers who can't wait to start scribbling.

The rules of sudoku are simple: Players fill in squares set in a nine-by-nine grid -- subdivided into boxes -- so that the numbers one through nine appear precisely once in each row, column and box. Part of its popularity stems from the fact that unlike its crossword cousin, sudoku requires neither encyclopedic knowledge nor a formidable vocabulary, making it a quick favorite among children and puzzle novices. Depending on the number and placement of the digits players are given to start with, puzzles range from easy to so complex that even mathematicians sometimes leave squares blank.

For the sudoku-weary, electronic and Web-based versions pose challenges that paper can't provide. They often offer a virtually unlimited number of games, more levels of difficulty and the ability to compete with other users. Finding newspaper puzzles too easy, Stephanie Lung, a 23-year-old from New York City, turned to the Internet. Ms. Lung, who works for the division of student affairs at Columbia University, found a sufficiently challenging "evil" puzzle off Websudoku.com and plays puzzles she prints off the site several times a week.

Companies say they are seeing strong interest in their mobile sudoku games. Howard Tomlinson, chief executive of mobile-game maker Astraware Ltd. estimates that there are dozens of different versions of mobile sudoku -- roughly 20 or so for Palm and Windows Mobile devices alone. Astraware's sudoku game, which costs $19.95 for billions of puzzles, is currently the eighth most popular of some 20,000 software downloads that are available for Palm devices.

While Astraware's software is geared towards pricier multipurpose gadgets, Sudokumo.com recently launched a mobile sudoku game that can be downloaded to most cell phones. The company, based in Guildford, U.K., says they have sold 7,500 games world-wide and are seeing growing interest from the U.S. Unlike the standard print sudoku, Sudokumo's mobile sudoku, available in packages of 10 puzzles for $2.50 or 30 puzzles for $5, has tutoring tools that help players identify the correct answer or tell them when they've entered the wrong one.

Briarpatch's board game, which is expected to be shipped next month, aims to solve a common complaint by players that they have to erase their guesses. It uses double-sided tile pieces that can be easily removed or stacked. Players use pieces of one color to set up the board to match a puzzle that appears in the game booklet -- or any puzzle they find in the paper. They then solve the puzzle with numbered tiles of the opposite color. The game, which will retail for around $20, is designed for travel with a tile-storing drawer.

Web sites where players can play and print puzzles, receive online help and trade tips are attracting millions of viewers as major newpapers post Web-based versions on their own sites. Sudoku's online migration is even launching the puzzle into classrooms. Marsha Ratzel, a 50-year-old math teacher at Leawood Middle School in Leawood, Kan., often breaks out sudoku puzzles she has printed off Edhelper.com. She says the site -- which sells sudoku-inspired puzzles that use pictures instead of numbers to help children identify, for example, endangered species -- has given her access to sudokus appropriate for all skill levels. Ms. Ratzel says she uses sudoku to teach her students how to approach a problem logically. "Sudoku helps them structure their thinking."

Still, the puzzle is already facing competition from a cohort of new Japanese puzzles, like kakuro, which some players find more rewarding because they require slightly more skill. And some players say they need to cut back in order to re-engage with society. Melissa Tanner, a 24-year-old psychology graduate student at American University in Washington, D.C., says her sudoku addiction has begun to irritate her boyfriend. When she pulls a puzzle out, "he just rolls his eyes," she says. "I'll try not to do it as much when we are hanging out so I can pay more attention to him."

First Published: February 9, 2006, 5:00 a.m.