Urbanization has deleterious effects on water quality and biota in stream systems. This project used caged bluegill (Lepomis macrochirus) to assess metazoan fish parasite communities in 2 urbanizing streams of the upper San Antonio River Basin, Bexar County, Texas. Field studies on Leon and Salado creeks were conducted during late summer in 1999 and 2000. Juvenile bluegill, obtained from a local aquaculturist, were held in cages for 10–22 days at middle and lower watershed sites to expose them to in-stream conditions and to allow parasite communities to establish. After removal from cages, fish were examined for metazoan parasites. In 2000, wild Lepomis spp. also were collected at study sites for parasite assessment. In both years, physical and chemical water properties were monitored at each site. Of the 120 fish examined for parasites, 96.7% were infected by at least 1 organism from among the 11 parasitic taxa observed. For caged fish, both diversity and equitability of parasite communities tended to be lower at the more eutrophic downstream sites; accordingly, parasite diversity and equitability were inversely correlated with nitrate concentrations. Ectoparasites were more prevalent in caged fish and endoparasites were more abundant in wild fish. An Ergasilus sp. copepod and a Posthodiplostomum sp. trematode dominated the ecto- and endoparasite faunas, respectively. This study suggests that assessment of watershed health can benefit from comparative cage studies of parasite community development involving sentinel fish species.