Our study details key parameters of the reproductive biology of the shrimp Palaemon longirostris, e.g., spatio-temporal variation of the sex-ratio, migration pattern, breeding period, fecundity, size at the onset of maturity, and egg volume. While P. longirostris population was very little studied in its distribution area, our fist aim was to complete that lack of knowledge and to provide a basis for understanding its population dynamic. Finally, our results were also compared with those reported from different studies on this species within Europe, in order to test the reported latitudinal cline of reproductive parameters in decapods. During a one-year survey, P. longirostris have been collected monthly across a large part of the Gironde estuary (France). Biometric measurements and determination of sex and maturation stages were carried out on preserved subsamples. Eggs were counted and measured for ovigerous females. In the Gironde, the breeding period was restricted from March to July, and related to water temperature. The size of the ovigerous females ranged between 7.1 and 14.6 mm cephalothorax length; according to their size, these females brought between 78 and 1391 eggs (547 on average). These results were compared with available data from the literature on French or foreign populations. For two reproductive parameters a latitudinal cline was noticed: e.g., an increase of the maximum females size and a restriction of the breeding period with latitude. Due to the weakness of available data on the reproductive biology of P. longirostris, it was very difficult to compare fecundity (number of eggs brought per female) and egg volume between localities. As expected fecundity could decrease with latitude, but this trend remains to be confirmed by more extensive studies on the species populations.