Saltmarsh Sparrows (Ammodramus caudacutus) are non-territorial, lack pair-bonds, and practice promiscuous mating behavior and obligate maternal care of young. Behavioral details of mating behavior and associated intermale aggressive behavior are poorly understood in the species. We report the results of an observational study of mating and agonistic behavior of individually marked breeding birds in New York. We witnessed 1,265 sexual and agonistic interactions within and among males and females from 1977 through 1985. We found no evidence of male mating aggregations, and male aggressive behavior was prevalent only in male-initiated sexual interactions. Females solicit matings from males during nest-building, but the behavior is inconspicuous and not associated with male aggression. Males spend the morning patrolling their home ranges, and chase or approach females they encounter anywhere in their breeding habitat. Males often concentrate patrol activity in the vicinity of nests under construction, but we found little evidence they know the location of most nest sites. Some males seek to forcibly mount females on the ground at any breeding stage, despite female resistance. Females thwart forced mountings in 57% of cases either by fighting with the males, or by uttering an aggressive call. When a more persistent male suppresses a female's resistance, she then crouches passively as he assumes a copulatory position on her back. We discuss this behavior in terms of female control of forced mountings, female choice of mates, and forced mating as a tactic of males that appear not to know the fertility status of females. Males have a large cloacal protuberance, which suggests sperm competition is strongly developed in the species. We caution that evolution of the unusual mating behavior in Saltmarsh Sparrows must be understood in relation to the different sexual behaviors of its closest relatives.