Abstract.1. Three experimental stream bed sections were established in areas of similar depth and current velocity to examine the effect of rock size upon the distributionof Baetis intercalarisand Orthocladiinae. Each section was composed of one rock size with the diameters (mm) of the three rock classes being 30–70, 90–140 and 160–190.2. Colonization was measured at 2 week intervals over an 8 week period, and these data were used to calculated abundance or the number of individuals per unit area of stream bed and density or the number of individuals per unit area of rock surface.3. Statistical analyses reveal that rock size was of secondary importance in determining Orthocladiinae abundance and density among the experimental bed sections. Rock size differences had a more pronounced effect uponB. intercalarissince on every sampling date both abundance and density increased with rock size.4. Rock size differences appeared to have little effect on the response of Orthocladiinae to silt deposition, whereas the reverse was true forB.interca