Presumably because it was defending its territory or its young, the beaver attacked a woman and her two daughters, along with their pet dog, whilst they were paddling in the Schwechat river.
Even when they ran out of the river and onto dry land it pursued them and bit their dog, taking a chunk of fur.
The family were enjoying the cool water when they saw what they assumed was a small tree trunk floating past. But as their dog sniffed it, it turned out to be an angry beaver, who immediately went into attack mode.
The mother took her children out of the water - but the beaver followed them onto dry land and thumped its tail on the ground in a threatening manner.
The family then decided to abandon their swimming trip and have said in future they will go to the local lido, instead of the river.
Beavers are vegetarians and attacks on humans are rare, but do happen when they're trying to protect their young, or they feel their territory is being invaded.
In German the word for beaver is Biber, prompting some wits to nickname the angry animal Justin, after the baby-faced Canadian singer.