The distribution and limnetic density of five species of Chaoborus larvae were determined in 26 lakes of the Matamek River System, Québec. The distribution of C. americanus is closely related to the absence of fish while C. punctipennis is associated with fish populations. The distributions of C. trivittatus and C. flavicans appear independent of the presence or absence of fish, but C. flavicans reaches its greatest density in the three lakes with the largest individual basins. A probable new species or subspecies of the subgenus Scadanophasma was found in three lakes. The standing crop of limnetic Chaoborus larvae is higher in lakes without fish than with fish.