1. Males of the digger wasp Philanthus basilaris defend and scent-mark small mating territories. There is a high rate of turnover of males on territories on both a daily and seasonal basis due, in part, to predation on territorial males and the large number of usurpations of territories. 2. The outcome of aggressive interactions on territories is determined by size differences among males, rather than by their status as resident or intruder. As a result, territory holders are larger on average than non-territorial males. The latter, however, remain in the area and attempt to usurp territories or replace residents that have abandoned their perches. Evidence suggests that larger territorial males are also less subject to predation by robberflies and conspecific females. 3. The type of mating system of this species resembles those defined as leks. Males established territories in groups of up to about 50. Nests of females were not found within aggregations of territories. There is a high potential for polygyny due to the lack of maternal care of offspring, dominance interactions which consistently favor larger males and result in intense sexual selection, and asynchronous emergence of females combined with potentially longlived males. Although large males are dominant, no body size related spatial structure to the aggregations could be identified. Aggregations of territories often occur in the same location from year to year, even though males live for only one season.