Population changes of Chironomids in a natural lake and in a man-made cooling reservoir were studied from November 1978 to February 1981. Biweekly collections were made at 20 stations in the lake and 16 in the reservoir. Water temperature and dissolved oxygen were measured at each station. On one occasion, samples of water and sediment were collected from each habitat. Glyptotendipes paripes, Chironomus crassicaudatus and C. decorus predominated among the collected species. Glyptotendipes paripes represented 63% and C. crassicaudatus 18% of the total collections made in the lake. The monthly compositions ranged from <1 to 97% for the former and <1 to 87% for the latter species. Their density in the lake varied from <100 to 6000/m2 and <10 to 1800/m2, respectively. Tanypodinae constituted 15% of the total larvae; their density fluctuated between 20 and 500/m2 over the study period. In the reservoir, G. paripes formed 80% of the total collections with monthly compositions of 40 to 97%, and density ranging from 300 to 8000/m2 . Chironomus decorus composed 16% of the total larvae. Its density fluctuated from 5 to 2000/m2. The spring and summer increases of G. paripes demonstrate significant correlations (r=+0.78 lake and +0.72 reservoir) with changes in water temperature. The consistently higher number of G. paripes in the reservoir than in the lake may have been due to a higher (3-5 C) water temperature in the reservoir. Significant spatial variations existed in the nitrogen, phosphorus and carbon concentrations in water and substrates in each habitat.