Migrating European eel (Anguilla anguilla L.) glass eels were collected annually in the Garonne Basin (France) during 1987-1989 and subsequently raised under freshwater conditions. In 1990 gonadal development was determined and after about 1 year it was re-examined to check if sex ratios had changed. Undifferentiated gonads were seldom found, and were mainly present in the youngest year sample 89-1+ (numbers refer to year of catch followed by freshwater age), This gonadal stage can develop directly into an ovary. Differentiation of male sex proceeded via an intersexual stage with male and female sex cells. Phenotypic expression of female gonadal sex was considered as being gonochoristic and differentiated. The female gonad gradually developed towards a fourth stage ovary, characterised by the presence of follicles containing oocytes arrested in the late prophase of meiosis. Yolk formation in oocytes was not observed. The intersexual progression to males was termed pre-maturational sex change. The testes of full grown males contained numerous tubules with spermatogonia in mitotic arrest, The percentage of intersexual eels in 1990 of the youngest year sample (89-1 +) was very high (91%). Most of these animals differentiated into males in 1991 (89-2 +). The percentages of undifferentiated and intersexual eels in the older year samples of 1988 and 1987 examined in 1990 were low, showing that sex differentiation in most animals had occurred after 2-3 years. The sex ratios expressed as males/(males + females) were 64% (88-2 +) and 76% (88-3 +); 84% (87-3 +) and 83% (87-4 +), Comparison of these sex ratios suggests that sex reversal did not occur, Selective mortality and migration, population density, feeding conditions and temperature are reviewed as possible factors controlling sex differentiation in European eel. (C) 1997 Elsevier Science B.V.