We quantified the impact of nesting and roosting House Sparrows (Passer domesticus) on nesting success of Cliff Swallows (Petrochelidon pyrrhonota) in colonies in western Arkansas in 2007 and 2008. Two sections of a large swallow colony under a bridge with House Sparrows were compared in 2007 to two sections with little House Sparrow usage. Nesting success of Cliff Swallows (percent of nests yielding at least 1 chick) was 61% in sections with low House Sparrow activity, significantly higher than the 30% in sections with high House Sparrow activity. House Sparrows defended a broad zone surrounding their nests from Cliff Swallow nesting attempts. We compared the proportion of nests used, clutch sizes, and brood sizes of Cliff Swallows in two colonies in 2008, one with and one without House Sparrow activity. In the colony without House Sparrow activity, 48% of old and new nests were used by swallows versus only 8% in the colony with House Sparrows. Swallow clutch sizes were similar in the two colonies, but swallow brood sizes in the colony with no House Sparrows were significantly higher, mean  =  2.3 nestlings per nest (mode  =  2; 75th percentile  =  3) compared to 0.8 nestlings (mode  =  0; 75th percentile  =  1) in the colony with House Sparrows. This suggests Cliff Swallows are less successful when House Sparrows are present in colonies.