In certain arthropod groups, including spiders, males seeking copulations may expose themselves to cannibalism from females that are larger and stronger than they are. Although old males are commonly believed to be particularly vulnerable to sexual cannibalism, virtually no data exist to back that supposition. Female crab spiders Misumena vatia (Thomisidae) regularly attacked prospective mates experimentally presented in pairs whose individuals differed in age. They usually attacked the older male; all but one of the seven males cannibalized in these encounters was the older member of the pair. These attacks paralleled a decline in numbers of field-observed males, probably a consequence of their increasing vulnerability. The females' responses appeared to result from differences in male behavior or condition, since virgin females, wild-captured adult (and probably previously mated) females and penultimate females all attacked males with similar frequency.