Swimmer’s itch, or schistosome cercarial dermatitis, continues to be a problem in Michigan. The most important intermediate and definitive hosts for these dermatitis-producing schistosomes are the lymnaeid snail, Stagnicola emarginata and the seasonally migratory common merganer, Mergus merganser (Aves: Anatidae), respectively. Two trends of swimmer’s itch are common on the larger recreation lakes in that area. On some lakes, outbreaks of swimmer’s itch begin during the first warm period, usually in late May or early June. On these lakes, the problem is greatly reduced or disappears by the end of July. Probable reasons for this decline include the death of older, infected snails and the absence of summer resident common mergansers. Because of the lack of definitive hosts, young snails do not get exposed to the schistosomes until the arrival of fall or spring migrants. The second trend occurs on lakes where swimmer’s itch is a problem throughout the summer. At these sites, female common mergansers and their broods continue to transmit infections to the young snails. These snails begin shedding cercariae in late June or early July, thereby continuing the malady. Factors that may influence the severity of swimmer’s itch include the level of infection in birds and the densities and location of both the intermediate and definitive hosts.