Abstract.Populations of the thalassinidean shrimpUpogebia pusillawere studied on tidal flats in the Northern Adriatic Sea. Biometric analysis showed a sexual dimorphism, especially in propodus size. Size frequency distributions revealed the presence of large animals during all seasons; recruitment by juveniles occurred in autumn. Moult intervals and moult increments were determined in the laboratory and used to generate growth curves which were compared with those calculated from size frequency distributions. The life span ofU, pusillais over 5 years. Ovigerous females occurred between March and September. Egg numbers were high in spring, lower in summer and increased with body size. The incubation time of embryos was 35 days; a female produces an estimated number of three egg batches during the breeding season. A total annual production of 994 kJ was estimated for a theoretical population of 100 animals; 13.5 % is spent for somatic growth, 31 % for egg production of females, and 55.2% is lost as exuviae. The population structures, growth and breeding patterns, as well as embryonic and larval development within theUpogebiidaeandCallianassidaeare discussed.