I studied the relationship of nesting habitat and the spatial distribution of Red-winged Blackbird (Agelaius phoeniceus) nests with the frequency of brood parasitism by Brown-headed Cowbirds (Molothrus ater). For all nests combined, parasitism was over 18 times more frequent in the upland habitat. It is unlikely that the low frequency of parasitism in the marshes was due solely to an increase in the incidence of aggregate nesting. For aggregated nests, parasitism was over nine times more common in upland habitat than in marshes. Because Red-winged Blackbirds are known to react to and deter cowbirds when cowbirds are detected near nests, I assessed whether the ability of Red-winged Blackbirds to detect cowbirds may contribute to the low parasitism frequency observed in marshes. Red-winged Blackbirds nesting in marshes more often reacted aggressively to female cowbird mounts 1 m from a nest than those in upland habitat, indicating that cowbirds are more likely to be molested near nests in marshes. In upland habitat the frequency of parasitism of unaggregated nests was three times greater than that of aggregated nests. These results suggest that Red-winged Blackbirds can defend their nests against cowbirds and successfully prevent brood parasitism and that the effectiveness of nest defense varies with habitat and the spatial distribution of nests. This study provides the first direct evidence that hosts can sometimes prevent parasitism.