In adult females of two euryhaline grapsid crabs, Cyrtograpsus angulatus and Chasmagnathus granulata, which coexist in a brackish coastal lagoon in Argentina, we studied the quantitative relationships between body size (carapace width, CW), wet weight (WW), dry weight (DW), ash-free dry weight (AFDW), and egg production (fecundity). The females of these two species have a similar average body size, gross chemical composition (percentage contents of water, ash, and organic matter), and a similar size and chemical composition of eggs. In both species, all size-weight relationships as well as those between egg number and either female size or weight could be described with allometric regression equations (power functions). Significantly different slopes in the size-weight regressions of these two species, however, indicate that there are specific morphometric differences. These are indicated also by a significantly higher average egg production in C. granulata, reflecting a proportionally larger volume of the body cavity in relation to CW Comparison of the fecundity of females with early or late-stage eggs shows, in both species, that there is a significant loss of eggs during the time of embryonic development (up to 32 % in C. granulata and 67 % in C. angulatus). The loss of developing eggs tended to increase with increasing female size in C. granulata and with decreasing female size in C. angulatus. In the latter species, a high incidence of mutiple limb loss (autotomy) has been observed as a common specific feature. Females which have lost at least half of their pereiopods show a significantly lower average fecundity and a higher varability in egg number as compared with intact individuals. The energetic investment of female crabs into egg production is compared with the relation between the amounts of organic matter deposited in the egg mass and that of the female body. On an AFDW basis, we measured a significantly higher ratio (21 %) in C. granulata than in C. angulatus (16 %). This reflects not only a greater fecundity in the former species; in addition, ia eggs have a slightly (but significantly) larger average size, DW, and AFDW. During the course of embryonic development, the egg volume increases in both species, due to an uptake of water, to about double the initial value. While organic matter (AFDW) decreases as a consequence of embryonic metabolism, there is a significant increase in the mineral fraction and the water content of the eggs.