The aims of the present study were (a) to compare sperm quality (percentage of motile spermatozoa, motility duration, density and fertility after cryopreservation) between precocious and normally maturing male European sea bass Dicentrarchus labrax, (b) to examine the potential of human chorionic gonadotropin (hCG) to increase spermiation in precocious males and (c) to examine the potential of hCG to induce spermatogenesis and spermiation in non-precocious 1-year-old males. One hundred precocious and 100 non-precocious fish were each randomly divided in two groups each: control (precocious saline-treated and non precocious saline-treated) and treated (precocious hCG-treated and non precocious hCG-treated). Treated groups were administered weekly with 1000 IU hCG kg(-1) body weight while control groups were injected with physiological solution. Milt volume produced, sperm concentration, motility duration and fertilising ability were assessed every week in each group. The effect of the hormonal treatment on gonadal development was examined based on the gonadosomatic index and testicular histology. The results demonstrate that sperm produced by precocious fish has characteristics (mean value of motility class, mean maximum motility duration, concentration and fertility after cryopreservation) similar (P > 0.05) to those produced by 2-year-old fish. Human chorionic gonadotropin treatment in precocious fish resulted in a significant increase (P < 0.05) of milt volume, without affecting sperm quality. In non-precocious fish, hCG treatment resulted in greater percentage of spermiation (P < 0.05) compared to non-precocious saline-treated group. At the end of the trial (three weeks), 29 out of 50 non-precocious hCG-treated fish were spermiating and, within these 23 produced > 200 mu l per fish of milt. No differences were observed in terms of sperm concentration, motility class, motility duration and fertilizing capacity due to hCG treatment in either precocious, or non-precocious fish. In addition, analysis of the testicular histology of fish that did not spermiate after hCG treatment, shows a significant (P < 0.05) enhancement of testicular development stages. The present study demonstrated that (a) precocious European sea bass males produce milt of comparable sperm characteristics to adult individuals, (b) treatment of non-precocious males with hCG induced spermatogenesis and spermiation and (c) treatment of precocious males with hCG enhanced milt volume without affecting other sperm characteristics, including fertilizing ability. (c) 2006 Elsevier B.V. All rights reserved.