Forest fragmentation may alter host–parasite interactions in ways that contribute to host population declines. We tested this prediction by examining parasite infections and the abundance of infective helminths in 20 forest fragments and in unfragmented forest in Kibale National Park, Uganda. Over 4 years, the endangered red colobus (Procolobus rufomitratus) declined by 20% in fragments, whereas the black-and-white colobus (Colobus guereza) in fragments and populations of both colobines in unfragmented forest remained relatively stable. Seven nematodes (Strongyloides fulleborni, Strongyloides stercoralis, Oesophagostomum sp., an unidentified strongyle, Trichuris sp., Ascaris sp., and Colobenterobius sp.), one cestode (Bertiella sp.), and three protozoans (Entamoeba coli, Entamoebahistolytica/dispar, and Giardia sp.) were detected. Infection prevalence and the magnitude of multiple infections were greater for red colobus in fragmented than in unfragmented forest, but these parameters did not differ between forests for black-and-white colobus. Infective-stage colobus parasites occurred at higher densities in fragmented compared with unfragmented forest, demonstrating greater infection risk for fragmented populations. There was little evidence that the nature of the infection was related to the size of the fragment, the density of the host, or the nature of the infection in the other colobine, despite the fact that many of the parasites are considered generalists. This study suggests that forest fragmentation can alter host–parasite dynamics and demonstrates that such changes can correspond with changes in host population size in forest fragments. Am. J. Primatol. 70:222–230, 2008. © 2007 Wiley-Liss, Inc.