A multidisciplinary hypothesis testing approach is adopted to investigate the intraspecific relationships, and venom evolution within the polytypic species, Crotalus viridis, in western North America. A molecular phylogeny is reconstructed from mitochondrial cytochrome b (678 bp) and NADH dehydrogenase subunit 4 (ND4; 669 bp) DNA sequence information. The phylogeny is not concordant with the conventional subspecies categories, but shows strong geographical structuring corresponding to the major geographic regions of the western U. S. The basal split gives rise to two lineages (5.1-6.4% sequence divergence) corresponding to haplotypes east and west of the Rocky Mountains. Within the western lineage, Crotalus viridis cerberus forms a sister group to the other western haplotypes, and appears to have a long, independent evolutionary history. Multivariate morphometric analysis also reveals regional structuring. Clear eastern and western forms are apparent, although these populations are not totally reproductively isolated. North-south clinal variation in morphology is found among populations east of the Rocky Mountains between Montana and Arizona. Among the western forms, there is clearly a zone of intergradation between the Great Basin and Pacific Coast forms, represented by a morphological cline in Idaho. Clinal variation was also found between the northern and southern Pacific Coast forms. Venom evolution is of interest in C. viridis, since C. v. concolor is the only subspecies of C. viridis to secrete a high toxicity PLA2 phospholipase (Concolor toxin) in its venom in adulthood. Isoelectric focusing of venom proteins revealed 14 variable bands, of which only one was unique (pI 8.52) to C. v. concolor. Principal coordinates analysis revealed three main venom types, corresponding to the Pacific Coast, C. v. concolor, and the remaining populations respectively. However, C. v. concolor tends to cluster with the latter group when the unique venom band is excluded from the analysis. 1 Phylogeny, rather than ecology, appears to be an important cause of geographic variation in both morphology and venom, as revealed by partial Mantel tests. Many characters are influenced by both phylogeny and ecology, however, probably because many causes of variation are intercorrelated. It is suggested that selection on venom composition probably varies according to the function of individual components. The systematics of Crotalus viridis complex is reviewed according the criteria of the general lineage concept of species. Combined evidence from the molecular phylogeny and morphology (and to a lesser extent venom) suggests the existence of three species, to be named Crotalus viridis, Crotalus cerberus, and Crotalus oreganus (including the subspecies C. o. oreganus, C. o. lutosus and C. o. concolor).