Humpback whales are being hunted for the first time in 40 years, leading to big protests from environmental groups.
There was a worldwide ban on hunting the creatures in the mid-1960s, but Japan is now allowed to kill the animals for scientific research.
Japanese hunters have gone to the South Pacific, aiming to kill up to 1,000 whales - including 50 humpbacks.
In the 1960s, the humpback was in danger of dying out and campaigners are angry that the hunt has been allowed.
Whale hunting is a controversial issue
"Humpbacks are very sensitive and live in close-knit pods so even one death can be extremely damaging," said a spokesman for the environmental group Greenpeace.
Japanese officials say that humpback numbers are now big enough to let the hunt go on.
"Taking 50 humpbacks from a population of tens of thousands will have no significant impact whatsoever," said one.
He said scientists will learn lots more about the creatures by examining the dead whales.