The numbers of cercariae that trematodes produce have been previously investigated in the laboratory but not in the field. I studied cercarial production of the bucephalid trematode Rhipidocotyle fennica in the freshwater unionid clam Anodonta piscinalis kept under natural conditions. Naturally infected clams (n=180) were collected and marked in early June 1996. Every 14 days, starting from the collection date and ending in October, these clams were taken to the laboratory where they were monitored for the emergence of trematode cercariae. Between monitoring dates, the clams were returned to the collection site. From a random subsample of infected clams (n=12), the number of cercariae produced was counted during every monitoring. Cercarial production started in late July. The proportion of infected clams releasing cercariae increased from late July to late August. The mean daily production also increased during that period from 140 to 10,400 cercariae, decreasing thereafter. The mean annual production was 290,000+/-26,000 (160,000-440,000) cercariae. Daily production in late August and early September was positively correlated with annual production. Two of the 12 clams appeared to be infected with both R. fennica and Rhipidocotyle campanula. The average annual productions of R. fennica and R. campanula cercariae in double infections were 181,000+/-58,000 and 73,000+/-29,000, respectively.