Palaemon elegans is a species of prawn new (since 2000) to the southern Baltic. The aim of this study was to find out whether there are differences in the sizes of individuals and in the reproductive traits of P. elegans inhabiting different areas of the southern Baltic Sea and to compare the data obtained with existing data for populations from other coastal areas. The present study was carried out in the summers of 2005 and 2007 in three areas differing in their hydrological conditions (primarily in salinity): Puck Bay (PB), the Vistula Delta (VD) and the Vistula Lagoon (VL), Poland. The maximum body lengths of these prawns (females = 60 mm, males = 41 mm), and body masses (1201 mg and 533 mg respectively) found in the southern Baltic were less than those reported from many other parts of the geographical range of this species. The increase in body mass with length was the smallest in the prawns inhabiting VL, and the maximum lengths they grew to were also shorter (females -52 mm and males -39 mm) than in PB. The numbers of eggs carried by P. elegans in the southern Baltic were smaller than those found in females in its native regions. Differences were also found in the numbers and sizes of eggs between prawns inhabiting different locations in the southern Baltic. It was concluded that the low salinity of the southern Baltic Sea (even below 7 PSU), while not preventing this prawn from reproducing, caused a shift to the production of fewer but larger eggs. The reproductive strategy that Baltic populations of P. elegans has adopted is one of the factors responsible for its very rapid and large-scale colonization of the southern Baltic Sea.