The present study describes the seasonality of egg production, the gonad cycle, the development of secondary sexual characteristics, and the size at first maturity of the caridean shrimp Betaeus emarginatus, a typical inhabitant of intertidal rock pools. A total of 1118 individuals (including 298 ovigerous females) was collected monthly from May 1993 to December 1994 in central-southern Chile. The sex ratio was 1.1: 1.0, and males reached larger sizes than females. Degeneration of the appendix masculina with increasing size was not evident, indicating that B. emarginatus is a dioecious species. The onset of sexual maturity of females was reached between 8.0 and 9.0 mm carapace length. Ovigerous females were present between May and December and were absent between January and April. The breeding season started during late autumn (May), and females with embryos close to hatching occurred between midwinter (July) and the end of the spring (December). On the bases of the embryonic development, duration of the incubation period, and gonadosomatic index, it is evident that B. emarginatus has a markedly seasonal breeding period with probably two successive spawnings per female per season. The absence of ovigerous females during the summer months (January and April) may limit breeding to periods with conditions most suitable for hatching larvae, thus increasing the survival of progeny.