Juvenile rainbow trout were examined during 1951–59 while in stream nursery areas and when migrating downstream. Their body-scale relationship was: total length (mm) = 4.06 + 2.10 scale radius (mm × 107) and relationship between length (mm) and weight (g) was W = 0.00001384L3.0426. Age composition of trout during autumn in the nursery areas averaged 68% age 0, 29% age I and 3% age II; trout in the downstream migration averaged 64% age I, 34% age II and 2% age III. Most downstream migration occurred between 21 May and 30 June, at night, on subsiding water levels and at water temperatures of 9–17 C. There was an association between numbers of juveniles in the nursery areas and numbers of subsequent downstream migrants.