Some aspects of the population ecology of Orchestia mediterranea, found in the estuary of Bou Regreg (Morocco), were studied over the course of 1 year. Histograms of cohort frequency and observations on reproductive activity showed seasonal variations all year long: O. mediterranea can be characterized as a multivoltine, semiannual species. By statistical analysis, associated with a procedure near successive maxima, the population structure was determined by identifying cohorts that are associated in a complex manner. All these characteristics are similar to those of O. mediterranea and Talorchestia deshayesei found in the Mediterranean Sea. The exceptionally high annual reproductive potential estimated from the fecundity (which varies with the age of females and with the seasons), the sex ratio, and the brood number of the successive cohorts from the same filiation, all in a nutrient-rich environment, suggests an r type of adaptative strategy.