Mated female brachyuran crabs store the spermatozoa within the spermatheca, pending internal fertilization. The present study reports on the metabolism of sperm stored in the spermatheca of the estuarine crab Melopograpsus messor. Results of enzyme assays indicate that lactate oxidation to pyruvate predominates over pyruvate reduction to lactate within the spermatheca of M. messor. The spermathecal contents also show considerable succinate dehydrogenase (SDH) activity which is greater than the fumarate reductase (FR) activity, the SDH/FR ratio being 5.7:1. Furthermore, manometric assays demonstrate that the sperm stored within the spermathecal lumen consume ambient oxygen, the rate of oxygen uptake per 1.000 × 108 spermatozoa being 0.108 ± 0.004 μl × h-1. The prevalence of lactate oxidation over pyruvate reduction and the occurrence of a high SDH/FR ratio within the spermatheca, together with the spermatozoal oxygen uptake, strongly suggest that, while stored in the spermatheca, the spermatozoa of M. messor rely on aerobic metabolism. The occurrence of high lipid and the relative sparseness in carbohydrates (with near absence of free glucose) within the spermathecal lumen further reinforce our suggestion for aerobic respiration by the sperm stored in the female reproductive tract.