We examined the changes in respiratory enzymes in response to oil effluent in freshwater mussels, Lamellidens marginalis. The mussels were exposed to two sublethal 1/4 (11.88 ppt) and 1/10 (8.55 ppt) LC50. After 30 days, the treated mussels were transferred to untreated fresh water and depuration (recovery) was followed for a further 30 days. At 7-day intervals, the respiratory enzymes SDH, LDH and MDH were analyzed in subcellular fractions (cytosol, microsome) from foot and gills and digestive gland of mussels. The results indicated that the oil effluent caused a disruption to the mussel's normal physiology. A decreasing trend in respiratory enzyme (SDH) activity in subcellular fractions of the digestive gland was observed predominantly in comparison to other fractions of mussel tissues for all oil concentrations tested, and 70-80% of respiratory enzymes were recovered from both recovery periods. From day 15, a decreasing trend in LDH was observed in the subcellular fractions of the digestive gland, foot and gill in all oil concentrations tested, and a 70-80% increasing trend was observed during both recovery periods. In parallel, a gradually decreasing trend was observed in MDH in the subcellular fractions of foot and digestive gland in all tested oil concentrations and 70-80% increasing trend was observed during both recovery periods. The results were analyzed for their significance.