We examined the influence of rock type and rock roughness on algal biomass (measured as chlorophyll a) during a survey of the Rocky Mountains streams (Colorado and Wyoming).  Rock roughness—a measure of texture and porosity—was calculated from a ratio of two surface area measurements.  Rocks with low roughness included shale, slate, and chert; these had relatively low algal biomass.  Rocks with high roughness included rhyolite, sandstone, and limestone. There was greater variation in algal biomass among rough rocks than among smooth rocks.  The high porosity of limestone and sandstone contributed to their high algal biomass.  Rock roughness did not always correspond to algal biomass particularly in intermediately rough rocks.