Venom yields and toxicity levels for Costa Rican snakes are reported. It was found that the sea snake, Pelamis platurus, possesses the most potent venom. Taking into consideration toxicity and venom yields, the fer-de-lance, Bothrops asper (formerly included in B. atrox), appears to be the most dangerous snake in Costa Rica and Lachesis muta, despite its impressive size, occupies a relatively unimportant position. The data show that in toxicity or neutralization experiments in which mice are used it is desirable to prolong the observation period up to 48 hours when employing crotalid venoms and up to 72 hours when using elapide venoms.