Data from two stream suggest that macroinvertebrate community structure exhibits a predictable, graded response to heavy-metal pollution. In the more heavily stressed sections of both streams, macroinvertebrates, other than tubificid worms and chironomids, were virtually eliminated from rock-rubble, riffle habitats. Midge larvae comprised 75 to 86% of all insects collected from the most grossly polluted stations and less than 10% of the insect commuities at the least polluted stations. The correlation coefficient for percentage of chironomids in relation to copper concentration was +0.93. The number of species of chironomids was inversely related to the proportional density of the family in samples. Caddisflies were numerically important at the intermediately polluted stations in both streams. Mayflies occurred only at the least polluted stations.