We studied the breeding biology of two Agelaius phoeniceus (Red-winged Blackbird) populations in south Florida. Red-winged Blackbirds in our study bred at low relative abundance in a wide variety of habitats, were rarely socially polygynous, and were highly aggressive toward intruders. The breeding season extended from March through August and coincided with the rainy season. Nesting periods were similar to those reported for other subspecies of Red-winged Blackbirds, but clutch sizes were small, with 2- and 3-egg clutches predominating. Nesting success varied from 28% on Sanibel Island to 63% on the Florida Keys. Both populations accepted a majority of artificial eggs into their nests, although cowbird parasitism has been rarely reported in south Florida.