The River Forss in Caithness, northern Scotland, was chosen as the site for investigation into the life history of the Atlantic salmon (Salmo salar L.). Work begun there in May 1939 was suspended in September of that year: during these months I made three visits to the river, 2-4 May, 4-IO June and 20-25 July; on the last two visits collections of young salmon, trout (Salmo trutta L.) and invertebrate fauna were made. The fish were examined for age, growth and food to provide data for a comparative study of the two species. Material was collected at two places on the river because it was thought worthwhile to make observations at two slightly different stations. Although results based on two visits are limited it is believed that they are sufficient to justify some conclusions that will be of interest, as there are few comparative studies of salmon and trout in the same waters. The River Forss (Fig. i), which is about 26 miles long, lies on Old Red Sandstone (Caithness flagstone series). The upper waters run through peat which overlies the sandstone and at Cnocglass there is, from morainic deposits in the river, evidence of granite. Throughout its course the topography of the river showed little change. At any place the riverbed was usually composed of stretches of coarse sand, coarse gravel and large stones; occasionally there were emergent rocks or flagstones paving the bottom. From Cnocglass to the sea the width of the river was usually about 10-12 m. and there appeared to be little variation in the rate of flow. The surrounding country changed, but not greatly, from the source to the mouth of the river, the upper waters running through peat, the reaches nearer the mouth through pastureland. The upper and lower parts of the river showed some difference in the chemical constitution of the water. It was possible to choose for intensive study of the growth and food of salmon and trout two stations similar in general characteristics but differing somewhat in landscape surroundings and chemistry of the water. The two stations were: (i) Cnocglass, a place on the upper waters, one of the chief spawning areas of the salmon. Here the river, which is about io m. wide and 50 cm. deep, runs through a cutting between peat-covered moraines, the banks being of grass and bracken. The bottom of the river is composed of loose gravel (fist-sized stones and smaller) and of head-sized stones, and in some places there are emergent rocks;. all are embedded in coarse sand. Vegetation consisted of patches of Fontinalis on the bigger stones and some Myriophyllum. The pH ranged from 6 6, to 7 , the alkalinity as CaCO3 was 49 mg./l., and the conductivity 15 i as reciprocal megohms. (2) Forss Bridge, a place 4 mile above Forss Bridge and Forss falls, and the lowest reach of the river in which salmon spawn. Here the river, I0-12 m. wide, and about 8o cm. in depth, runs through grazing land and there is no peat. The river bottom is similar to that at the Cnocglass station. The pH ranged from 7-o to 8-2, the alkalinity as CaCO3 was 8i mg./L., and the conductivity was 208. The difference between the two stations was slight, but the pastureland surroundings, the higher values for pH, alkalinity and conductivity at Forss Bridge suggest more favourable conditions.