Growth and smoltification were followed during the first year of life in 14 full sibling families of sea trout, Salmo trutta m. trutta L., originating from the Vistula stock. Until January, the families were reared separately, and then together after they had been PIT tagged. The degree of smoltification was evaluated in the second spring. Smolts were noted in each family. The percentage of these, which ranged from 14 to 66%, depended on mean fish length in October (r2 = 0.338) and January (r2 = 0.346). In either October or January the fish that smolted next spring were, on average, longer than those that did not, but this difference was not manifested by the bimodality of length distribution, and it did not allow predicting the number of smolts based on division of distributions of fish length in autumn or winter into Gaussian components.
