Seasonal changes in the population structure and patterns of vertical distribution and migration of the dominant chaetognath, Sagitta elegans, in a fjord off Puget Sound were determined with samples collected at approximately semi-monthly intervals from November 1972 to November 1973. S. elegans bred continuously from March through August, with major recruitment in June and July. Seasonal changes in the vertical distribution of the population and in the proportion of the population that vertically migrated reflected the stage of maturity and size of the individuals. Small chaetognaths were non-migratory and distributed in the top 100 m. Stage II individuals were dispersed throughout the water column; some did migration was evident. Breeding stages were restricted to a layer between SO and 100 m during the day and migrated to the surface layer at night, enhancing the probability of successful reproduction.