Aspects of growth and reproductive cycle of the prawn Palaemon adspersus were studied in Alfacs Bay (Ebro delta, western Mediterranean) . This species shows a marked sexual size dimorphism. Sexual differences related to length-weight were noted. The sex-ratio observed was significantly biased towards females in the study area. Cohort analysis of the size-frequency distributions suggests the occurrence of a main recruitment period in the study area during August and September. The estimated von Bertalanffy growth parameters suggest rapid growth for both sexes. Ovigerous females were recorded between January and September. Those carrying eggs about to hatch appeared from March. Some of the early specimens appearing in the benthic population in summer (yearling individuals) had already spawned at the end of the reproductive period, together with prawns from the previous generation (wintering individuals).