Your cat is biting for a reason that makes sense to him, even if his behavior alarms you. If he's never bitten before, something has triggered this new behavior. He could be in pain from illness, or reacting to the presence of another pet, human or even an unfamiliar sound he perceives as a threat. If the cat is a new arrival in your home, his habit may be a holdover from something he's learned in the past.
Often a cat bites as a part of playing roughly, especially if he's less than 2 years old. Kittens learn fighting and biting behavior from encounters with their littermates or mothers, grabbing each other in mock battles, swatting and nipping, but not inflicting injury. Through their rough-and-tumble play, they learn to sheath their claws, and not to sink their teeth into the other cat. Orphaned cats or those that were weaned early may never have distinguished the difference between rough play and real fighting. Lonely cats lacking stimulating playtime may alleviate their boredom with aggressive behavior. Cat owners who think it's fun to encourage their felines to attack human hands or feet are giving Fluffy the wrong message.
No matter how angry or scared you are, never ever physically punish a cat that bites. Hitting, slapping, kicking or throwing the cat can injure him, and will not stop the biting. It will cause him to become fearful of you, and he may become even more aggressive. If he's been biting as a form of what he sees as play, the play could turn rougher. Although some cat owners use a squirt of water to get a cat to stop what he's doing, a thorough vet exam first will help rule out or pinpoint any medical issues behind the biting behavior. Read on for some other tips on training your cat not to bite.