The seasonal variation in abundance, biomass and vertical distribution of nauplii and copepodites of Temora longicornis in the Bornholm Basin was studied from March 2002 to May 2003 to understand the overwintering, spring development and life cycle of this species in the Baltic Sea. The analysis of the life cycle by means of stage structure, copepodite length and stage duration revealed that T. longicornis produced 5 to 6 generations yr–1. The species overwintered in low abundance as an active, slowly developing generation with adults appearing from February/March onwards. The onset of the spring bloom in April triggered reproduction and initiated the first spring generation (G1) with a strong rise in nauplii abundance. The stock biomass increased in May with the occurrence of the copepodites of G1 and remained high during the succeeding generations G2 and G3 until August. The stock was distributed in the upper, brackish 30 to 60 m of the water column. In summer, copepodite stages concentrated in the cool intermediate water during daytime and migrated to the surface at night. This seasonal submergence explains the persistence of T. longicornis in the Bornholm Basin throughout the year because the average temperature experienced by the population was low (5 to 14°C) compared with the unfavourable warm surface temperatures (>20°C). The study further showed that the timing of the spring biomass increase of the species is associated with the maturation of the first generation and can vary considerably between years. A delay of ~1 mo was observed in 2003 when environmental conditions during the cohort development were unfavourable. We conclude that the spring development of T. longicornis is complex and depends not only on prevailing temperatures, but also on spring bloom timing and post-bloom food availability.