Microdistribution, life history, and secondary production of two species of Hydropsychidae were studied in the outlet of the eutrophic Lake Belau (Schleswig-Holstein, Northern Germany). A total of 3,647 larvae were collected at 11 sampling sites with a box sampler: 1,995 larvae of Hydropsyche angustipennis and 1,652 larvae of H. pellucidula. 133 adults of H. angustipennis and 130 adults of H. pellucidula were caught by 12 swimming emergence traps. Annual mean hydropsychid density in the stream was 202 ind. m −2. Highest densities were observed at gravelly substrates with 1,080 ind. m −2 of H. angustipennis and 780 ind. m −2 of H. pellucidula. At sites with fine sand, sand, and peat less than 11 ind. m −2 of both species were found. Both species were univoltin and during almost the entire year larvae of the instars 3 to 5 appeared in the stream. The main growth of H. angustipennis was observed from May to June, just before the larvae emerged. Looking at H. pellucidula two growth phases could be identified, the first approximately in the fourth month of life (September) and the second between May and August of the following year. During 10 months the larvae of H. anguistipennis showed an almost constant mean individual weight of 0.4 to 0.5 mg DM (dry mass) and the larvae of H. pellucidula of about 1.5 mg DM. In the investigated stream mean annual biomass of H. angustipennis was 115 mg DM m −2 and production 484 mg DM m −2 yr −1 (P/B = 4.2). H. pellucidula had a mean annual biomass of 231 mg DM m −2 and a production of 428 mg DM m −2 yr −1 (P/B = 1.9). Both species emerged from May to September with maxima in May and August. Biomass of emerged H. angustipennis was 94 mg DM m −2 and of emerged H. pellucidula 116 mg DM m −2. The ratio for emerged biomass to the larval secondary production E/P amounted to 19.3% for H. angustipennis and 27.0% for H. pellucidula.