To address the lack of information on the ecomorphology of young of the year juveniles (0 +) freshwater fishes, despite its importance to understanding geographical variation in life history, we compared the morphology of 0+ juvenile roach Rutilus rutilus from eight sub-regions of the River Great Ouse basin (U.K.). We tested for variation due to water retention structures, which could act to isolate upstream stocks from the general population. Differences in mensural characters between the eight different regions appeared in ventral fin length and eye diameter only, which both were strongly affected by variable relative growth during early juvenile development; thus the discrepancies between regions were not due to geographical location. Morphological development in roach ≥ 15 mm standard length (SL) consists of two different intervals: during the first (≅ 15–40 mm SL), in contrast to adults, several mensural characters continue to develop allometrically, either positively or negatively, suggesting a continuance of the larval period. Whereas, in the second interval (> ≅ 40 mm SL), all the mensural characters except head length and eye diameter grew close to isometrically with body length, and the rate of variation in relative growth became as strongly limited as that previously reported for adults. In contrast to previous studies, which are based on few specimens, we suggest this second interval to be the true start of the juvenile period.