Are all Buddhists vegetarian?

Most if not all Buddhists I know are vegetarian or vegan. Thinking about the precepts this is not surprising - one should abstain from killing. However is this universally true in all Buddhist traditions? Are there some Buddhist traditions that emphasize vegetarianism more than others? For instance I believe the Tibetans were originally pastoral farmers which wouldn't lend itself to vegetarianism naturally. So is Buddhist vegetarianism universal, specific to tradition or culture, or a lot more flexible than that?