1.1. The proximate (water, protein, lipid, mineral ash) and elemental (carbon, nitrogen, phosphorus) composition of eggs of the freshwater prawn Macrobrachium rosenbergii (de Man, 1879) (Crustacea, Decapoda) were determined in relation to embryonic development. Eggs were sampled at four intervals during development, ranging from day 1 (newly spawned) to day 16 (about to hatch).2.2. Egg dry mass, lipid and carbon content all decreased during development as reserves were utilized; at the same time both water and mineral ash content increased. Although the mean content of both nitrogen and protein declined during development these were not statistically significant due to a high variance. Egg phosphorus content remained constant throughout development.3.3. In the lipid fraction triacylgycerol fatty acids provided the major fuel, although phospholipid was also utilized. The most important substrates were 16:0 and 18:1 fatty acids.4.4. There were strong suggestions of an initial increase in egg lipid content during the first 5 days of development, though this was not statistically significant and the source of the extra material is unknown.