A larval population was studied (1994-1996) on the Zschopau river, Saxony, Germany. In 325 larvae head widths and wing-sheath lengths were measured, and the abdominal segments that were partly or completely covered by the wing-sheaths were counted. Within the population, there was much size variation throughout the yr. This is interpreted in terms of co-occurrence of the univoltine and semivoltine cohorts. The semivoltine F-0 larvae were larger than the univoltine of the same stage. There are more 6 6 than $ $ in the larval population.