Freshwater mussels were exposed to several concentrations of DDT (2,2-bis(p-chlorophenyl)-1,1,1-trichlorethene) and dieldrin (hexachloroepoxyoctahy-dro-endo, exo-dimethanonaphthalene) in natural lake water and reconstituted distilled water under continuous flow and constant temperature conditions. The mussels concentrated DDT approximately 2400 fold and dieldrin 1200 fold in lake water. They concentrated DDT about 1000 fold in distilled water. The concentration of pesticides in the mussels reached equilibrium with the level in the water faster in lake water than in distilled water and the pesticide also had a shorter half-life in the mussel in lake water. The half-life of dieldrin was 4.7 days in lake water compared to 12.6 days for DDT in lake water. The pesticide concentrations were highest in the digestive and reproductive tissue and low in the muscle, mantle, and gill tissues. The concentrations were very low in the marsupia in tests run in distilled water but were almost as great as those in the digestive and reproductive tissue in lake water.