In recent years populations of howler monkeys (Alouatta palliata) in southeastern Mexico have decreased substantially due to the transformation and loss of natural habitats. This is especially evident in the Santa Marta mountain range, Veracruz, Mexico where several studies have evaluated the impact of fragmentation on howler monkey populations in order to propose management programs for their conservation. The conditions generated by fragmentation likely change the rates of parasitic infection and could decrease howler survival. In this study, gastrointestinal parasite species richness, prevalence, and egg density of infection were determined in howler groups inhabiting five forest fragments at the Santa Marta mountain range. Two hundred and seventy-eight fresh fecal samples were collected between October 2002 and April 2003. Three parasite species were found during the dry and the wet season in all forest fragments sampled: one unidentified species of Eimeriidae; Trypanoxyuris minutus (Oxyuridae); and Controrchis biliophilus (Dicrocoeliidae). Both the prevalence of T. minutus and infection density for all parasites differed between seasons and fragments (the largest fragment consistently differed from other fragments). Host density, distance to the nearest town, fragment size, fragment shape, and total basal area of food trees explained parasite prevalence, but each species had a different pattern. Although parasite richness was lower, prevalence and density were higher than values reported for howlers in conserved forests. These results suggest that the establishment of biological corridors and animal translocation programs must take into account the parasite ecology of each fragment to avoid higher infection rates and preclude potential consequent mortality. Am. J. Primatol. 72:539–548, 2010. © 2010 Wiley-Liss, Inc.