A study, by immunoelectrophoresis and immunodiffusion of the shared antigens in venom from 21 species of Viperinae and Crotalinae, representing 10 different genera, has produced a number of results of taxonomie significance which are summarized schematically in the dendrogram shown in fig. 1. For the most part these immunological data confirm the accepted classification of the Viperidae which is based on comparative morphology and biogeography. Some results, on the other hand, such as the isolated position of Atheris, need confirmation with more material. Some of the conclusions reached in this study, however, appear to be of significance for an understanding of the phylogeny of the group. Amongst the Viperinae, the genera Echis and Cerastes are apparently closely related to one another and more similar to the genus Bitis than to the genus Vipera. This latter genus is clearly composed of three groups of species with V. russelli occupying an isolated position. Amongst the Crotalinae, the genus Agkistrodon is quite distinct but Trimeresurus flavoviridis did not differ immunologically from the three species of Bothrops studied. It would appear that the study of the shared antigens in snake venom is of taxonomie value both at the species group and the family levels. Occasionally, however, antigenic proteins in snakes venom are known to evolve rapidly, especially in isolated populations, and consequently the presence of a relatively small proportion of shared antigens should not necessarily be taken as evidence of a lack of affinity between two taxa.