Why did my rat bite me?

Rats are social creatures and NEED a companion of the same species, Rats with out another Rat companion a majority of the time become aggressive, skittish, and depressed. This is probably the cause of the biting incident, along with him being in a new home where he is uncomfortable and scared. He needs to get used to your scent which could take days, weeks, or some times even months with pet store Rats. Get him a Rat companion, then leave them both alone for a couple days to allow them to get used to their new environment.