A band of approximately 52-53 kDa, corresponding to glial fibrillary acidic protein (GFAP) was detected by immunoblot techniques in brain tissue of a marine fish (Dentex dentex L). The aim of this study was to quantify the density of GFAP during the life cycle of a marine fish in the wild. The levels of GFAP immunoreactivity were determined at two different stages of development: juvenile (body weight range: 180-360 g) and sexually developing-mature specimens (body weight range: 2329-5550 g) from a natural population in the Western Mediterranean Sea. Our results indicated a negative correlation between the immunoreactivity of GFAP and weight in the sexually developing-mature group of fishes (r=-0.785, p<0.05, n=8). These findings might be of relevance in understanding the role of GFAP in the development of the central nervous system and demonstrate that GFAP down-regulation in D. dentex brain tissue during development is a natural physiological phenomenon.