The lipid composition of Crassostrea gigas was analyzed during the reproductive phase in natural as well as under artificial conditions. The lipid content increased and accumulated in the gonads, but to a higher extend in the naturally conditioned animals. The percentage of neutral lipid in total lipid of the gonad plus mantle was stable, high (>70%) and equal under both conditions, underscoring that the lipid reserves were preferentially located in that organ. The composition of the polar lipid classes was stable with little variations. The fatty acid (FA) composition of the neutral lipids in all organs is influenced by diet. However, there is a different response according to organs. A high dietary impact occured in the digestive gland whereas the muscle was less affected. The polyunsaturated fatty acid (PUFA) level of the neutral and polar lipids in the gonads changed little despite the dietary conditions but the respective proportions of (n-3) and (n-6) PUFA differed drastically as a result of diet composition. There was clear evidence in all organs for a specific accumulation of 22:6(n-3) and 20:5(n-3) in the polar lipids under both conditioning diets. The proportions of 22:6(n-3) and 20:5(n-3) from neutral and polar lipids of oysters conditioned artificially were significantly lower than of those that were naturally conditioned. Copyright (C) 1999 Elsevier Science Inc.