The effect of beaver, Castor canadensis (Kuhl), on an eastern brook trout, Salvelinus fontinalis (Mitchill), population was studied in the vicinity of five beaver dams in the headwaters of Sunkhaze Stream. The beaver dams were found to present a serious obstacle to trout movements but they were not a complete barrier. Estimated unit-area production of bottom organisms was poor in the beaver ponds but the beaver had so increased the bottom area that total production of the stream section was probably more than doubled. Forage fishes appeared to be somewhat more abundant in the beaver ponds than in the open stream and constituted an unexpectedly high percentage of the food of 6- to 10-inch brook trout taken in the ponds. Measurable but not serious water-quality reduction was found in and below the beaver ponds.