In order to analyze the evolutionary behavior of the cobra venom cytotoxins, their probable tertiary structure was predicted using computer graphics. The 41 amino acid sequences known show that the major evolutionary changes have taken place in two particularly exposed areas of the molecular surface. In each area, neighboring residue positions seem to have evolved interdependently, but there is no obvious interdependence between the two areas. Indeed, the relative evolution of these two areas prompts a subdivision of the sequence set into four groups. According to the known cytotoxin circular dichroism spectra, one of these four groups could be characterized by a difference in molecular secondary structure. Since the two variable areas have functional associations, it is suggested that their evolution may be governed by a target with several similar binding sites.