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1. 1. The distribution of ceramide aminoethylphosphonate (CAEP) in microsomal membranes obtained from different tissues of the bivalve mollusc Diplodon delodontus was determined. 2. 2. The concentration of CAEP reached from 9 to 19% of the total microsomal polar lipids, depending on the kind of tissue. 3. 3. Palmitic acid was the main fatty acid in the ceramide moiety, followed by stearic and eicosamonoenoic acids. 4. 4. Artificial membranes were prepared with microsomal phospholipids or phospholipids plus sterols, with and without the addition of CAEP. 5. 5. It was shown that the phosphonate confers minor mobility to the membranes. This effect is more effective when the membrane contains the natural sterols and the phospholipids are unsaturated.

1. 1. The composition of lipids and fatty acids of male and female gonads of D. delodontus at different sexual stages were studied. 2. 2. The main differences were found in the percentages of triacylglycerols, cholesterol esters and phospholipids. 3. 3. Female gonads with few immature gametes contained more triacylglycerolds and less phospholipids than male gonads in similar conditions. 4. 4. Spermatogenesis increased the sterol esters of male gonads. Oogenesis increased triacylglycerols and sterol esters of female gonads. Non-fertilized eggs contained a high percentage of triacylglycerols and a low percentage of sterol esters. 5. 5. The fatty acid composition of gonads was low in saturated fatty acids. The unsaturation was scattered in 16 : 1, 18 : 1, 20 : 1, NMI 22 : 2 and polyunsaturated acids of ω6 and ω3 families. Only 20 : 1 exceeds the 10% of total fatty acids. Similarly to other Diplodon species, the ratio of ω3 : ω6 acids was low and typical of fresh water molluscs. 6. 6. Oogenesis increased the proportion of palmitoleate, α-linolenate and oleate: stearate ratio. The change of the fatty acid composition was mainly produced in the triacylglycerols. The changes were not due to unsaturated fatty acid biosynthesis in the gonads. However, oocytes were able to elongate palmitic and stearic acids.