One juror said the number puzzle helped keep her "mind busy".
A drugs trial has been abandoned in Australia after several jurors admitted they had spent much of their time playing Sudoku in the courtroom.
Judge Peter Zahra aborted proceedings after the jury forewoman admitted she and four others had been playing the popular puzzles to fight off boredom.
The problem was discovered when some of the jurors were observed writing notes vertically rather than horizontally.
Sudoku involves completing a grid of numbers in the correct sequence.
The three-month trial had cost taxpayers more than A$1m (US$945,000) and the two accused men faced possible life sentences.
One of the jurors explained that the puzzle had helped keep her "mind busy" as she listened to repeated testimonies from the witness box.
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The right to be tried by a jury of your peers is a fundamental human right. To abdicate those freedoms and duties because it is boring is pure selfishness
"Some of the evidence is rather drawn out, and I find it difficult to maintain my attention the whole time," the juror told the Australian Associated Press.
Lawyers had presumed the scribbling they could see jurors doing was note-taking.
"We actually all thought they were quite a diligent jury," lawyer Robyn Hakelis told ABC local radio.
"The judge had made many comments about what a good jury they were, how they were taking copious amounts of notes."
There is no legal action that can be taken against jurors for doing puzzles during a trial, so they will face no penalty for their behaviour.
A fresh trial is expected to begin in a few weeks, once a new jury has been called.