A large size variation amongst life histories for streamโdwelling Atlantic salmon Salmo salar was found and the relative effect of life histories on size varied over time. As early as December (age 0+ years), fish that later smolted at age 2+ years were significantly larger than fish that did not smolt at age 2+ years. In contrast, there were no mass differences at age 0+ years between fish that would mature or not at age 1+ years (October). The mass differences between smolts and nonโsmolts persisted until smolting, and differences between mature and immature fish first appeared in May (age 1+ years). Following September (age 1+ years), there was also a significant interaction between smolting and maturity. Previously mature and immature age 2+ year smolts were not significantly different in size, but immature age 2+ year nonโsmolts were much lighter than mature age 2+ year nonโsmolts. Based on mass differences, the apparent โdecisionโ to smolt occurred c. 5 months before (winter, age 0+ years) the decision to mature (late spring, age 1+โyears). In addition to strong seasonal growth variation, sizes of freshwater Atlantic salmon were largely structured by the complex interaction between smoltโage and maturity.