Host-parasite interactions between Red-rumped Caciques (Cacicus haemorrhous) and Giant Cowbirds (Molothrus oryzivorus) were studied at 52 nests in the Atlantic Forest, Misiones, Argentina. Cowbird eggs (1–6) occurred in 27 of 38 cacique nests (71%) found during the egg stage. Giant Cowbird eggs were white and unmarked (85%), or marked and spotted over pale buff (15%); the marked eggs somewhat resembled host eggs, but were twice as large. Four host and three parasite eggs were found punctured and broken. Only three cowbird nestlings were observed, and resembled oropendola (Psarocolius spp.) nestlings more than those of Red-rumped Caciques. Botflies (Philornis spp.) infested cowbird and cacique nestlings, but there was no evidence of a cowbird-cacique preening mutualism.