The relative roles of genetic (GSD) and temperature sex determination (TSD) were investigated in turbot, Scophthalmus maximus, with the objective of producing all-female stocks for aquaculture. Androgen treated males (ATM) and oestrogen treated females (OTF) were produced after testing various 17 alpha-methyltestosterone or 17 beta-oestradiol treatments delivered orally (3 or 5 mg steroids/kg food) from 35 days postfertilization (dpf) during either 500 or 700 degree-days respectively. GSD was analyzed by checking the sex ratio of families (n = 33) obtained by crossing control parents with other control parents, either ATM or OTF. TSD was investigated on 8 families reared at three temperatures (15 degrees C, 17 degrees C or 24 degrees C) from weaning to 100 dpf. All the steroid treatments were efficient in producing either 100% mate or 100% female populations demonstrating the production of androgen sex-inversed males and oestrogen sex-inversed females. The results of the GSD analysis suggested that most of the data fit to a ZZ/ZW model of sex determination. However, some crosses deviated from this model and other minor factors may be involved. interestingly, minor effects of temperature and a limited but significant interaction between family and temperature have been observed. In conclusion, the present data suggest that a major ZZ/ZW genetic model is involved in turbot sex determination. Unexpected sex ratios deviating from this model are discussed, including a possible minor effect of environmental factors, such as temperature. (C) 2009 Elsevier B.V. All rights reserved.