Seasonal variations in the occurrence and maturation of the nematode Cystidicoloides ephemeridarum (Linstow, 1872) were followed in brown trout, Salmo trutta fario L., in the Homolsky Brook (the Elbe River basin) at Velke Brezno, North Bohemia, the Czech Republic, during the period of 14 months (from April 2000 to May 2001). Nymphs of the mayfly Ephemera danica Müller were found to be the main intermediate host; the degree of infection of mayflies was relatively high throughout the year [total prevalence 74%, intensity 1–115 (mean 2) nematode larvae], but showed distinct seasonal changes with maximum values in May and minimum values in June. Other, less important mayfly intermediate hosts in this locality were Habroleptoides modesta (Hagen) and Ecdyonurus dispar (Curtis); the latter represents a new intermediate host record for C. ephemeridarum. The loach, Barbatula barbatula (L.), was found for the first time as the natural paratenic host of this nematode. In this locality, C. ephemeridarum had no distinct generations, but gravid females laid eggs throughout the year; seasonal changes were only quantitative. The main period of oviposition was from May until October (maximum in August). New infections were acquired by trout all the year round, but mainly from September until May of the next year. The parasite’s seasonal maturation cycle was induced by ecological factors, particularly water temperature and seasonal changes in the populations of mayfly intermediate hosts.