Leioproctus muelleri sp. nov. is described from south-western Australia, and a new subgenus, Ottocolletes, is established to contain it. The species is remarkable for the relatively enormous head and mandibles of its males. Both sexes were encountered at nesting sites in a dry, winter-wet depression where pollen-laden females were observed entering holes in the ground and males flew close to the ground surface. Four nests in which females were constructing and provisioning cells were occupied also by single males. Patrolling males stopped frequently to inspect burrow entrances and occasionally entered but usually backed out quickly apparently repelled by occupant males. Skirmishes observed and photographed at nest entrances revealed that fighting males grasp their opponent's head in their mandibles. Male macrocephaly in this species almost certainly is associated with mate-guarding or progeny-guarding behaviour. Observations are presented also on nest architecture, pollen provisions and dormant larvae that survive inundation of the nesting area during winter.