By Roland Buerk
BBC News, Tokyo
The men say the meat was in a box marked as containing cardboard
Two Greenpeace activists are on trial in Japan after trying to expose alleged embezzlement in the whaling fleet, which is heavily government subsidised.
The pair took a box of whale meat from a courier company warehouse and displayed it at a news conference.
They said it was evidence whalers were smuggling meat to the black market with the knowledge of the authorities.
But an investigation into their allegations was dropped and they were charged with trespass and theft.
The two men face up to 10 years in prison if convicted.
Junichi Sato and Toru Suzuki are pleading not guilty, although they admit taking the whale meat in northern Japan in 2008.
They said the meat, which weighed 23kg (7st 1lb) and had been in a box marked "cardboard", had been stolen by a fleet crew member for the black market.
Greenpeace said it had evidence others had also smuggled whale, disguised as personal baggage, and accused them of defrauding Japanese taxpayers with the approval of officials from the fleet.
The fleet operator said staff were given small amounts of meat as a bonus for spending months in the Southern Ocean.
Prosecutors initially agreed to investigate the activists' allegations, but then arrested them.
Greenpeace hopes the case will spur opposition to whaling in Japan.
The Japanese fleet kills hundreds of the animals during annual hunts in Antarctic waters.
Its ships have been involved in repeated clashes with campaigners in recent weeks.
Commercial whaling has been banned since 1986 - but Japan says that although whale meat ends up in shops and restaurants, it is actually conducting scientific research.