This study explored the risk of predation by trout (non-host predator) on adult hairworms Gordionus chinensis. Ten percent of trout (18 out of 187 trout) ingested adult hairworms in a Japanese headwater stream of the Totsu River. All hairworms were ingested by trout that had also ingested their insect definitive hosts (camel crickets: Tachycines elegantissima and T. asynamorus). Trout had never foraged hairworms directly in the two field experiments. These results suggest that hairworms were ingested by trout via predation on their hosts. Large trout ingested hairworms more frequently than smaller trout did. The larger trout were more abundant in the main stem compared with the upper tributary, which might suggest spatial heterogeneity of predation risk for adult hairworms within a river system.