Among young largemouth bass 48 mm. long, living with a "normal" population of other fish, including large predators, the removal of one pectoral fin, or both ventral fins, in July resulted in significantly decreased survival from that time to Nov., during which time they grew 50 mm. on the avg. The magnitude of the effect is estimated as an increase of 0.24 in the instantaneous mortality rate, which is 50% of the mortality rate of unmarked fish. From Nov. to the following May the marked individuals suffered an additional 50% excess mortality, which increased the instantaneous rate of 0.83 for unmarked fish to 1.24 for marked ones. The total differential of 0.65 corresponds to a survival rate, among marked bass, 52% as great as that of unmarked bass. During their 2d summer of life, larger piscivorous fish were absent from the pond and there was no excess mortality among the marked bass as they grew from 95 mm. to 178 mm. During the winter period mentioned above, mortality was heaviest among the larger bass of both the marked and the unmarked groups. A similar greater vulnerability of large individuals might explain the fact that, at the end of the previous summer, the survivors of the marked individuals were significantly smaller (4.7 mm.) than the unmarked ones. However, it is also possible that marking affected the rate of growth directly. Two-yr.-old bluegills, marked when 138 mm. avg. length, suffered very little if any extra mortality during one growing season, and were as vulnerable to capture by fishermen as were unmarked ones. The growth of the marked and unmarked bluegills was the same. Similar results were obtained from a less well controlled expt. using redear sunfish 175-185 mm. long.