A 20-m section of stream riffle was covered with black plastic to decrease light intensities and reduce algal standing crops. After four weeks, chlorophyll a concentrations were near zero and Baetis tricaudatus nymphs were far fewer in the experimental riffle than in control riffles above and below the shaded area. Also, Baetis adults, reared from pre-emergent nymphs collected in the experimental riffle, were significantly smaller than reared adults from the control riffles. After three months, we removed one half of the black plastic from the experimental riffle. The algae and B. tricaudatus nymphs recolonized the open section of the experimental riffle within 11 days. These results suggest that algae are an extremely important resource for B. tricaudatus, a collector-gatherer. Reduction of epilithic algae did not greatly influence Simulium, a filter feeder. Other macroinvertebrates that apparently were not influenced by the algal reduction included Asellus, Gammarus, Hyallela, Dugesia and Chironomidae.