Stagnicola emarginata were collected from 3 northern Michigan lakes and examined for larval trematode infections. The structure of the trematode community was then compared with 2 previous studies conducted on Douglas Lake in order to determine what changes had taken place over a period of more than 50 yr and to examine the possibility of using trematodes as bioindicators of environmental quality. Species richness was reduced by half (from 16 to 8 species) from the first study conducted in 1936, along with a decrease in the overall prevalence from 61% to 13%. No new species had colonized the lake, and the same 8 trematodes were also the only species found in 2 other lakes. The decline in prevalence was most severe in trematodes that used gulls as definitive hosts, whereas species that used ducks as hosts increased slightly. The decrease in species richness and prevalence of infection over time may reflect increasing human impacts on lakes. However, because of patchy host distributions, the use of larval trematodes as bioindicators of environmental change requires further investigation.