Daily increment formation was demonstrated in the sagittae of the three‐spined stickleback,Gasterosieus aculeatusL., under both laboratory and field conditions. The onset of daily increment formation coincided with the day of hatching. The main sources of inaccuracy in age determination resulted from difficulties in resolving narrow increments in the sagittae of fish held at low temperatures and the increasing incidence of checks with age. The advantage of daily increments for investigating growth in this species was demonstrated from a comparison between increment‐derived age‐at‐length data and length‐at‐capture data, since the latter significantly under‐estimated early growth rate within