Two types of spawning sites were utilized by fallfish (Semotilus corporalis) within our study area on the Mill River, Amherst, Massachusetts: single nest stream-edge sites under overhanging vegetation, and midstream sites that had 2-3 nests a few meters apart. Spawning behavior differed markedly in two respects from closely related nest-building minnows. Communal spawning with varied numbers of both sexes taking part occurred over gravel nests. Males and females did not clasp during spawning. A highly structured male social hierarchy operated at spawning sites. Males used the parallel swim, lateral display, headbutt, and other aggressive display patterns to establish status within the hierarchy. Females did not exhibit aggression or nest-building. Dominant males built nest mounds by gathering stones from as far as 5 m from the nest. Once a male established the dominant position in the hierarchy he initiated spawning by swimming to the nest with a stone in his mouth and dropping it, or by positioning himself slightly downstream and "rushing" forward onto the nest. The ritualization of nest-building movements to initiate spawning apparently is unique to the fallfish among the North American minnows that build gravel nests. Other fish species present around nesting male fallfish did not interfere with the spawning activity, but aggressive activity by non-nesting male fallfish often disrupted spawning acts. Male size and the outcome of aggressive interactions determined the status and relative reproductive success of male fallfish.