The age of both yellow and silver eels was detd. by markings on scales and otoliths and correlated with length and wt. A small minority of [female] [female] become silver when older and larger, possibly due to a more rapid rate of growth. Female silver eels had spent 12.27 (range: 9-19) yrs. in fresh water, were 60.8 (47-95) cm. long, and weighed 410 (210-2040) g. Male silver eels formed a small % of the catch. They had spent 9 (7-12) yrs. in fresh water, were 39.9 cm. long and weighed 112 g. Most [female] eels migrate when 54-60 cm. long although their ages varied. The length-wt. relationship does not follow the cube law, and the change in shape and substance with length is different in yellow and silver eels. A silver [female] eel < 58.06 cm. long is heavier than a [female] yellow of the same length, while above this length a silver [female] is lighter than a yellow [female] . The length-wt. relation in yellow and silver eels shows no definite change with age; in yellow eels it is higher in the latter part of the yr. Growth appears more rapid in continental waters than in Windermere and to be affected by environmental factors.