The pesticide, methoxychlor, in a carrier solution of methanol and Trition X-100, was applied to periphyton communities grown under conditions of continuous flow laboratory bioassay using a natural water supply. A replicated experiment with controls and pesticide levels of 0.125 ppb, 0.25 ppb, 0.5 ppb, 1.0 ppb and 2.0 ppb was designed. A parallel experiment with controls utilized carrier solution levels equivalent to the 0.25 ppb and 2.0 ppb pesticide levels. In both experiments, dry weight and organic weight production (g/sq m/day) progressively increased from the controls through the 1.0 ppb levels and decreased from the 1.0 ppb level to the 2.0 ppb level. Analysis of chlorophyll a production (mg/sq m/day) showed an initial increase in productivity from the control to the 0.25 ppb pesticide level after which there was a progressive decrease through the 2.0 ppb level. The parallel experiment which dosed the carrier solution by itself indicated a progressive decrease in chlorophyll a production from the control to the 2.0 level. The data suggest that the methanol/Triton X-100 carrier solution by itself as well as methoxychlor in the carrier solution affected periphyton communities by inhibiting algal species or chlorophyll production, and selected for a nonalgal community.