As the total rock surface area per unit area of stream bed differed significantly between riffles, comparisons between riffles for some species of Trichoptera could only be made in terms of numbers per unit rock area surface area rather than the conventional numbers per unit area of stream bed. The variance – mean density relationships of nine species of Trichoptera were constant on different sizes of rocks and from samples taken with a cylinder sampler in riffles of a brown-water stream. The number of individuals per rock was significantly greater on large rocks (surface area > 200 cm2) compared with intermediate and small rocks (40–200 cm2). The numbers of individuals per rock of Hydropsyche simulans and Glossosoma velona were greater on algae-covered rocks, whereas the numbers of Cheumatopsyche sp., Lepidostoma pluviale, and Micrasema sp. were greater on moss-covered rocks.