The effect of salinity on embryonic development of Chasmagnathus granulata and Cyrtograpsus angulatus (Crustacea: Decapoda: Grapsidae), the most common estuarine crabs of the southwestern Atlantic Ocean, was studied by means of an in vitro culture technique. Ovigerous females and, consequently, embryos inhabit Mar Chiquita, a coastal lagoon characterized by wide daily and seasonal changes of salinity. Larvae of both species are exported to the open sea and do not tolerate low salinities. Chasmagnathus granulata embryos synchronously completed development in vitro from 12 to 44 psu, exhausting yolk reserves, having an aspect and mobility similar to those observed in vivo. In spite of antibiotic treatment, C. angulatus egg cultures became infected by filamentous bacteria and fungi after a few days, and the embryonic development could not be completed. Nevertheless, in vitro cultures allowed study of salinity effects on early embryos of both species. One day old embryos of C. granulata and C. angulatus were able to develop at salinities between 12 and 40 psu, although the mortality varied widely among broods from different females. Outside this range, embryos of C. angulatus died; those of C. granulata could survive at 6, 44, and 50 psu although the development was arrested. Tolerance of embryos of both species to extreme salinities (3 and 44 psu) was acquired after two or three days following egg extrusion. Successful hatching rarely happened in C. granulata embryos cultured in vitro, and this was probably related to the lack of some female's contact-factor during the 48 h before hatching. The possibility of a correlation between the development of salinity resistance, the formation of egg external membranes, and the behaviour of ovigerous females is discussed.