A series of limnological surveys carried out from 1956 until 1965 were made in the vicinity of the Dickerson Plant of the Potomac Electric Power Company (located below the confluence of the Potomac and Monocacy Rivers) by the Limnology Department of the Academy of Natural Sciences of Philadelphia. These surveys were made at various stages of growth (i.e., increased generating capacity) of the power station and at both high and low water levels of the river. Cluster analyses were made of the fish data in order to provide a rapid and explicit means of analyzing the data, particularly relationships among aggregations of species. A secondary purpose was to measure the redundancy of information obtained by analysis of other groups, namely algae, protozoans, aquatic insects, and other invertebrates. On the basis of the cluster analysis of data on occurrence and distribution of fish, no changes in the environment of the Upper Potomac River can be ascribed to the operation of the Dickerson Plant of the Potomac Electric Power Company. This conclusion agrees with other results obtained by analyses of protozoan, algal, and insect species. Jaccard coefficients among aggregates of species of fish are appreciably higher than for other groups of organisms, indicating the relative mobility of fish compared to the other organisms. Comparing the fish data with that obtained for other groups of aquatic organisms indicated that there is much information redundancy when all groups of aquatic organisms are studied simultaneously. This redundancy is considered desirable since the results obtained with one group of organisms regarding pollution loads is much more significant if the same results are obtained with other groups of organisms.