The mating systems of seven previously unstudied members of the colletid bee genus Hylaeus Fabricius and one of Hyleoides Smith are described. Male mating tactics can be categorized as territorial (perched males defend flowers or other sites that attract receptive females) or non-territorial (patrolling males search for receptive females at flowering plants). The four species in which some territorial males occur are characterized by: 1. grappling fights among males for preferred perches; 2. territorial control by larger males; 3. the possession of prominent spines or other projections on the venter of the abdomen in larger males; and 4. the occurrence of some males that are as large as, or larger than, the largest females of their species (the ‘large-male phenomenon’). In contrast, the four species that lack territorial males are distinctive in that males: 1. do not engage in grappling contests; 2. lack abdominal weaponry; and 3. are smaller than the largest females of their species. In addition, we searched for the large-male phenomenon in museum collections of four species of Hylaeus that exhibit male abdominal spines (presumed to be the weapons used by territorial individuals) and two other species that lack these attributes (presumed non-territorial patrolling species). The results tend to support the sexual-selection-for-fighting-ability hypothesis, which argues that the evolution of unusually large males is a selective consequence of aggressive male—male competition for access to mates. The limitations of the present data set as a comparative test of this hypothesis are discussed.