Reproductive traits of an estuarine crab, Neohelice (= Chasmagnathus) granulata (Dana 1851), were compared between individuals living in contrasting habitats (mudflat and saltmarsh) of the same population in the brackish coastal lagoon of Mar Chiquita, Argentina. In both habitats, most measures of egg biomass decreased during embryogenesis, including total dry weight (DW) and organic matter (measured as ash-free dry weight, AFDW) per egg, the contents of ash, carbon, hydrogen and nitrogen (per egg and as percentage values of DW), the energy content (estimated from C; both per egg and per mg DW), and the C/N and C/H mass ratios. Egg size, wet weight (WW), and water content (in µg and % of WW), by contrast, increased significantly during the time of embryonic development. These parameters reached significantly higher final (near-to-hatching) values in mudflats than in saltmarshes (egg volume 0.0249 vs 0.0233 mm3; WW 36.5 vs 28.8 µg; water content 30.7 vs 23.2 µg per egg or 84.2 vs 80.2% of WW, respectively). Fecundity and reproductive effort did not differ significantly between habitats. Habitat-specific differences in the water content and size of crab eggs are discussed in relation to small-scale local variation in environmental conditions.