Corbicula manilensis Philippi were probably established in EI Capitan Reservoir by 1962. Relatively few live specimens, and no empty shell' were observed during 1964. Empty shells were lirsl observed during the summer of 1965 and outnumbered living specimens by 1967. Live clam populations increased from 234 x 10<sup>5</sup> individuals during the summer 1964 to a peak of 10,816 x 10<sup>5</sup> during January 1966. They declined to 6,744 x 10<sup>5</sup> during the summer 1967. These fluctuations were partually attributed to water level changes and reservoir stratification. Thermal and chemical stratification limited the depth distribution of Corbicula to the shallow, aerated depths during stratified periods. Their distribution extended to all depths following prolonged periods of artificial destratification. Corbicula densities appeared positively correlated with sediment mean partical size.