We have tried to show that algal and protozoan species frequently have complex requirements that may rival those of the higher organisms. Aggregations of these species (communities) in nature are constantly undergoing a replacement of species as environmental conditions change and competitive advantage shifts. Although this is no different from successional processes of higher organisms, there has been a tendency to view these changes as chaotic and fortuitous. This is probably due to the short time span involved and the difficulty of determining spatial relationships in microbial communities. It is now becoming increasingly evident that the same ecological principals which apply to higher plants and animals are also valid for algae and protozoa. Microbial communities have structure which is maintained despite succession, and other homeostatic mechanisms are operative. We must stop regarding microbial communities as random aggregations of species thrown together by chance and regard them as structural communities produced by the same ecological determinants as communities of higher organisms. Although many types of pollution exist, the response of algal and protozoan communities is generally rather simple, a reduction in complexity usually evidenced by a reduction in the number of species present. If one regards the universal microbial species pool as an information system about the natural environment, then species will always be available to replace those eliminated by normal environmental change. Any change outside the normal range will cause a disequilibrium situation not easily rectified by the usual homeostatic mechanisms. In short, the information system of the universal species pool does not fit. This results in a simplification of the system with the usual ecological consequences (e.g., increased oscillation). In an earlier paper, one of us (Cairns) defined pollution as "any environmental change which alters the species diversity more than 20% from the empirically-determined level for that particular locale". Although one might dispute the percentage, the genral definition still seems applicable. With a finite ecological base and increased pressures on this base, we must learn to use it wisely. This means (a) developing standards to protect the system, and (b) learning enough about its operational prerequisites to manage it well. At present we are sadly deficient in both areas.