The evolution of social structures and reproductive behaviours leading to actual spawning in the Mediterranean dusky grouper, Epinephelus marginatus, was studied in the Medes Islands Marine Reserve on the Spanish coast (NW Mediterranean) from February 1995 to October 1996. Density, size, sex, colour patterns, relation the bottom, swimming activity and site occupation were monitored by visual census surveys at monthly and daily scales. The main observed socio-behavioural trends were in part comparable with some patterns reported in tropical groupers, although differences exist, which might in part be related to the temperate conditions of the northern Mediterranean sea. The density of dusky groupers in the area was low in winter, and maximal in summer, from late June to late August (up to 8-fold increase); numbers rised in spring, before the beginning of reproductive activities which led to spawning by mid August, and dropped rapidly afterwards. During this long reproductive period (about 2 months), dominant males (recognized by their highly specific colour pattern) established territories in which they displayed aggressively towards neighbouring males and numerous smaller females (the species is proterogynous); the observed sex-ratio was approximately 1:7 (sexually active males versus adult females). Territoriality of males is the base of the social structuration of the reproductive population; in addition, behaviours typical of the pre-spawning period include repeated evening gathering of females in open water, and the appearance of specific colours patterns. This overall socio-behavioural pattern changed in the evenings when spawning was observed (16th - 21st of August), the males patrolling a reduced territory in which females would come to mate; spawning behaviour itself was described in Zabala et al. (1997).The complex socio-behavioural structure leading to reproduction in the Dusky Grouper should be considered in fishery management, since it could hardly develop under strong human pressure. In this respect, the contribution of Marine protected areas is determinant for the reproductive success of this species in the NW Mediterranean basin.