We used electrophoretically identified material to assess the geographical distributions, life cycles and interspecific hybridization of two sibling species of the Mysis relicta species group (sp. I and sp. II) in the northern Baltic region. In the Gulf of Finland, sp. I prevails in inshore waters and sp. II in the open sea; the distributions overlap in the outer archipelago zone. In the Gulf of Bothnia, only sp. II was found in the southern part (Bothnian Sea), whereas the two species coexist throughout the northerly Bothnian Bay. Both the local and large-scale distributions are salinity-related, but salinity alone does not explain the differences. The two species exhibit different patterns of geographical variation in their life histories. In strict sympatry in the north they have identical two-year life cycles with winter breeding. Further south (Gulf of Finland), sp. I exhibits a predominantly one-year winter-breeding cycle, whereas sp. II breeds throughout the year. The patterns comply with the concept of a great phenotypic flexibility and environmental control of life history characteristics in the Mysis relicta group, and make a contrast to the stable life cycle of the congeneric M. mixta. F1 hybrids between the two M. relicta group species were found at a low frequency (0.6%) in the Bothnian Bay, but not in other areas of sympatry.