During experiments in 1945 and 1946 in which sulfamerazine in the food was found to be effective in the treatment of furunculosis in brook trout (Salvelinus fontinalis), it was observed that fish grew faster on untreated food and that when the dosage rate was heavy they did not eat all the food and showed a dislike for it. To determine the effect of sulfamerazine treatment on growth and to learn whether the drug injures various trout species, brook trout and rainbow trout (Salmo gairdnerii) were tested in 1946 and brown trout (Salmo trutta) in 1947. The daily dosage rates chosen were 0, 5, 10, and 15 grams of the drug per 100 pounds of fish (0, 11, 22, 33 grams per 100 kilograms).