Competition among males is a major force shaping sexual selection. We investigated the male mating strategy of the widow spider Latrodectus revivensis, a desert species with an annual life cycle. Based on morphology of the sperm storage organs reported for female Latrodectus, we predicted that males should guard sub-adult or virgin adult females. In a natural population, we found that males were generally monogamous, cohabiting longer with sub-adult females approaching the final molt than with adult females. Nevertheless, both the duration and timing of male cohabitation were highly variable. Males were found with females from a few days before or after female maturation to over two months after maturation. Maturation of males and females peaked in spring, with a second, smaller peak in summer. Adult males that matured in spring were larger than those maturing later in the summer, while for adult females the pattern was reversed. We suggest that large males of L. revivensis that mature in spring maximize reproductive success by mating with virgins. Late males will gain greater reproductive success from mating with large, late-maturing females, but the scarcity of these females in the population at this season may make opportunistic mating with non-virgin females a viable strategy.