Despite high densities in an upstream lake, the herbivorous calanoid copepod, Diaptomus leptopus, is extremely rare in oligotrophia Eunice Lake in the Coastal Range Mountains near Vancouver, British Columbia. In situ experiments conducted in 1979 and 1980 tested the hypothesis that competition from zooplankton species resident in Eunice Lake prevents immigrant D. leptopus from colonizing the lake. Polyethylene enclosures, each holding 29,000 1 of lake water, contained all experimental treatments. Experiments in 1979 exposed a standard density of D. leptopus to all Eunice Lake zooplankton species at lake densities (control), to all Eunice Lake species at reduced densities (low density), and to all Eunice Lake species except one of Daphnia rosea (Daphnia-removal), Diaptomus kenai (kenai-removal), or Diaptomus tyrelli (tyrelli-removal). Improved performances of D. leptopus in non-control treatments was evidence for competition in controls. Measures of performance included density, number of eggs per female, and adult size. Similarity between D. leptopus performances in control, Daphnia-removal, and kenai-removal treatments plus relatively high concentrations of D. leptopus nauplii in the tyrelli-removal treatment suggested that D. leptopus did not compete with species other than D. tyrelli. However, similarity between tyrelli-removal and low density treatments in concentrations of D. leptopus nauplii confounded competition between D. leptopus and D. tyrelli with diffuse competition from several Eunice Lake species combined. D. leptopus overlapped in vertical distribution and seasonal cycle more with D. tyrelli than with other species. Differences' among experimental treatments in algal size compositions did not conclusively show whether zooplankton partitioned food resources. Experiments in 1980, designed to separate D. tyrelli competition from diffuse competition, did not provide any evidence of interspecific interactions. By increasing zooplankton metabolic efficiencies, cool temperatures in 1980 may have virtually eliminated, competition for algal foods. / Science, Faculty of / Zoology, Department of / Graduate