The ornate wrasse Thalassoma pavo is a small protogynous fish of the family Labridae living in shallow rocky habitats and Posidonia beds. High temperatures in summer are crucial for the sexual maturation of this subtropical species, which is by far more common in the Southern parts of the Mediterranean Sea. In the Ligurian Sea T. pavo was nearly unrecorded, notwithstanding the Ligurian fish fauna is quite well known since the nineteenth century. This labrid has been recorded at Camogli and Portofino in 1902 and 1911, respectively (Tortonese, 1965). No other occurrence is known until 1985 (Relini et al., 1988). Since 1991, adults and juveniles of T. pavo were frequently observed in several localities along both the eastern and western Rivieras. The increased occurrence of the ornate wrasse in the Ligurian Sea in recent years seems to support the hypothesis of Astraldi et al. (1995) of the establishment of true populations of warm-water species in the Ligurian Sea. To verify such hypothesis, a systematic study of the Ligurian populations of T. pavo started in 1996.