Sphecius grandis is a univoltine, colonial wasp. Females mate once and are sexually receptive when they emerge in July and early August. Males generally emerge earlier in the summer than females. The opportunity for each male to acquire mates is a function of the number of females emerging during his lifetime and the number of competitiors that are active when the females emerge. I determined a mating opportunity index (MOI) for each male in an aggregation of wasps for three separate summers, and correlated the MOI of individual males with their emergence date. The MOI of a male estimates the potential contribution that timing of emergence makes to his reproductive success. In 1984 males emerging near the mean of the male emergence distribution had the highest MOI. These males emerged between one and two weeks prior to the mean female emergence date. However, in 1981 late emerging males had the highest MOI. In 1983 there was no significant difference in MOI among males. As a result of between-year variation in female emergence schedules and in the duration of male lifetime, the selection pressures influencing male emergence time vary between years.