The Rhopalophthalmus mediterraneus population of the Guadalquivir Estuary (SW Spain) was sampled monthly from January 1998 to December 1999. Density, population structure, brood size, and standing stock were recorded during both years. The estuarine population of R. mediterraneus was concentrated in the outer 20 km of the estuary. There were considerable seasonal changes in the mysid abundance, with minima observed in winter and maxima in spring and summer. Such spatio-temporal distribution seemed to be related to estuarine environmental conditions: salinity was primarily responsible for the horizontal distribution of the species within the estuary, while its seasonal trend was determined primarily by water temperature. The population was characterised by a clear dominance of adults on all sampling dates and sites, with females (66.8%, on average) significantly more abundant than males (23.8%, on average), and the proportion of adult males and females changed significantly among sampling sites, being higher at the outer area (ANOVA; SNK test, P < 0.01). Gravid females were collected throughout the year, with maximal reproductive activity occurring in spring and summer. The population has a complex life cycle, being probably multivoltine and iteroparous. The brood size was significantly dependent on female size and season. The mean length of adults of both sexes varied negatively with temperature: smaller sizes occurred in summer.