From this study it is certain that brook trout (Salvelinus fontinalis) in excellent growing condition often inhabit and successfully complete their entire life-history in waters of distinct alkalinity. Since Dr. Coker has shown that they likewise live in water that is acid or neutral, it is quite evident that this factor, at least as expressed by the hydrogen-ion concentration, has little to do with the toleration of a given habitat by brook trout. Just what are the limits of trout as far as hydrogen-ion concentration is concerned is still a question. Experiments of Jewell and Brown (’24) and of Brown and Jewell (’26) on this and other species seem to indicate that fishes, given a choice, prefer that hydrogen-ion concentration found in the water in which they have been living. In any event, they did not react to hydrogen-ion concentration at all sharply. Further it appears that streams of the same physiographical nature and of a similar hydrogen-ion concentration, but of a different temperature, have a very different fauna. It would seem then that in the region studied, brook trout certainly tolerate and probably prefer water so cold (8 to 18 C) as to exclude, at least during the summer, most of the fishes characteristic of small streams. Since the hatchery records show that brook trout grow more rapidly in the warmer waters, those streams of maximum temperature for brook trout, but still exclusive of most other stream fishes, are the best trout streams. Such streams are often in such a delicate balance that a slight increase in the temperature of the water, such as is inevitable subsequent to the clearing of the land, renders them unfavorable for brook trout. The temperature difference therefore between a very good and a poor trout stream is very slight. Further studies of the environmental factors of brook trout waters are needed before very definite conclusions can be drawn, but it is certain that the toleration of hydrogen-ion concentration extends over more than a limited range including both the acid and the alkaline series, and that there appears to be a rather small but critical temperature difference between good and poor trout streams.