A total of 2,568 specimens of 3 species of ostracod, namely Herpetocypris sp., Ilyocypris sp., and Cypria reptans Brohnstein, 1928, was collected between November 1994 and April 1995 from the Ceresina Canal, a tributary of the River Brenta (northern Italy), and examined for larval helminths. Larvae of Neoechinorhynchus rutili Müller, 1780 were found in the hemocoel of C. reptans. The prevalence of infection ranged from 8.57 to 14.51%. Most larvae were in the cystacanth stage (a new geographical record for this cystacanth). Not more than 3 larvae were encountered in a single host. Among 48 aquatic alder fly larvae of Sialis lutaria collected over 6 mo, together with the crustacean, no infected larvae were encountered. Specimens of Gasterosteus aculeatus (Pisces: Gasterosteiformis) collected with the arthropods from the same site were found to be parasitized with adult N. rutili. Fish stomach content analysis showed ostracods to be a major food item of three-spined sticklebacks in the study area during much of the year.