I read somewhere...that cats cannot be vegetarian as they need meat in their diet to maintain proper health. However you can add vegetables to supplement their usual cat food if you wish. In the wild, when a feral cat eats a mice, he also ends up eating the mice's diet which is made up of vegetable/fibre. Do consult your vet if you insist on pursuing a vegetarian diet as well as do more research in the internet. However I am very certain that cats will not do well on a purely vegetarian diet