In brooding mollusc species that incubate larvae before their release in seawater, the biology of gametes remains poorly explored. The present study describes some biological characteristics of sperm of the European flat oyster, a native species that has been over-exploited in the past and is nowadays classified as an endangered species in Europe. Flat oysters were collected by divers in the Rade de Brest (Finistere, France), during its natural reproduction period. Gonadal pH is acidic (6.31 +/- 0.10). Spermatozoa are clustered in spermatozeugmata, an acellular structure in which the sperm heads are embedded. After their transfer in seawater, spermatozeugmata have a mean diameter of 64 +/- 3 mu m and they release free spermatozoa for a mean duration of 21 +/- 3 min. Immediately after their release, the mean percentage of motile spermatozoa was 48.5 +/- 12.6%. At 10 min after dilution in seawater, movement of spermatozoa was no more observed. Biological characteristics of European flat oyster sperm are compared to those observed in the Pacific oyster, regarding the unique reproductive behaviour of the former species and the role of spermatozoa transfer played by spermatozeugmata. The present results aim to improve the knowledge of reproduction and natural recruitment processes, support conservation and restoration measures and favour the establishment of management protocols of gametes and larvae in this endangered species.