From 1960 to 1965, 12,151 winter flounder (Pseudopleuronectes americanus) were tagged with Petersen tags at 21 locations off Massachusetts. Returns through 1971 totaled 4,440 or 36.5%. Ratio of females to males at tagging was 2.3. Movements north of Cape Cod were relatively localized and confined to inshore waters, whereas south of Cape Cod, flounder seasonally dispersed in a southeastward direction generally beyond the territorial limit; movements appeared to be related to water temperature. Little mixing occurred between Georges Bank and inshore areas. Growth of fish north of Cape Cod, south and east of Cape Cod, and on Georges Bank was described by “Walford lines” developed from tagging data. In 1964 apparent rates of exploitation, natural mortality, and annual total mortality were 0.201, 0.273, and 0.474, respectively, for fish released in Vineyard and Nantucket Sounds.