Gloating Keith Mann threatened a laboratory director as he left the dock for plotting a raid on Wickham Laboratories.
Just seconds after being handed community service for the break-in, Mann fronted up to police boasting 'I've won'.
He them smugly leaned towards the lab's technical director Chris Bishop and whispered: 'Your trouble is only just beginning. You will need to look under your bed.'
His threatening behaviour meant he was quickly hauled back before the court and jailed for six months for contempt of court.
Judge Richard Price at Portsmouth Crown Court said: 'I will not have people leaving my court saying that sort of thing. This was a serious threat and a serious contempt of court.'
Mann, 38, of Buckland Road, Poole had been given 230 hours community service after being found guilty of breaking into the labs, which carry out legal testing on animals, and stealing 695 mice in December 2003.
Mann's co-defendant Melvyn Glintenkamp, 42, of North Drive, New Milton, was handed a 170-hour community punishment order after being found guilty of conspiracy to burgle. He had allowed Mann to keep the mice in a caravan at his home.