Ecological distributions of chlorinated hydrocarbon residues in Lake Poinsett, South Dakota were studied from June, 1967 to June, 1968. Components of the ecosystem analyzed were water, bottom sediments, zooplankton, benthic algae, crayfish, aquatic insects and fish. Concentrations of aldrin, p,pˈ‐DDD, p,pˈ‐DDE, p,pˈ‐DDT, dieldrin, endrin, heptachlor, heptachlor epoxide, lindane, methoxychlor and toxaphene were determined by gas chromatography and thin‐layer chromatography. DDT and its metabolites, DDD and DDE, were the highest residues detected in all trophic levels examined. Heptachlor, heptachlor epoxide, aldrin, dieldrin and lindane were present in the majority of sample types, while neither endrin nor methoxychlor was detected above analytical confidence limits in any sample. Toxaphene was present in four fish. DDT and metabolites increased in percentage of total residue, while the ratio of DDT to DDD plus DDE decreased with increase in trophic level. The ratio of the epoxide form of heptachlor and aldrin to the parent compounds increased with higher trophic level. Water had the lowest total residue reported. Bottom sediments and crayfish had 18 times the residue level of water, while zooplankton and benthic algae showed a 37‐fold increase over water. Total residue in fish averaged 790 times that of water and aquatic insects had the highest increase over water (7300‐fold). Higher fat content of fish was correlated (r = 0.40, d.f. = 72) with higher insecticide levels. Analysis of variance showed residue levels tended to increase with age (P < .05). The correlation between age and fat was 0.66 (d.f. = 59). No significant difference was found by analysis of variance between sexes or between fall and spring collections. Residue levels in Lake Poinsett water were similar to levels reported for other areas, but fish displayed a much lower magnification over water than has been reported in the literature.