The reproductive success of a male is thought to be a function of the number of females he fertilizes. Nevertheless, various naturally occurring male behaviours seem to reduce dramatically the probability of obtaining additional matings. Such behaviours are expected when mating opportunities are limited for males and when males compete strongly for females or fertilizations. In widow spiders, males cohabit in females' webs, engage in long courtship displays, often lose the tip of one or both emboli (copulatory organs) inside the female's genital tract during copulation and are occasionally cannibalized by the female. We investigated conditions that may favour the evolution of a high-investment strategy of mating in white widow, Latrodectus pallidus, males. We followed male movements between females in a patchily distributed population of white widows and estimated the minimum number of males with which field-caught females mated from the number of embolus tips found in their genital tracts. The encounter rate of males with females was low with less than 20% of the searching males reaching a female. Females, in contrast, frequently mated with more than one male (up to six). We suggest that low encounter rates with females, coupled with high intrasexual competition, drives the evolution of a strategy of high investment by males in a single mating, as was observed in the white widow spider.