The Migjorn artificial reef (MAR) was deployed in the summer of 1990 and is made by 48 concrete blocks set 30 m deep on a sea bed of sand with a small proportion of Posidonia oceanica (L.) Delile meadow. Visual censuses of blocks on both substrata were carried out during the months of February, May and September in 1992 and 1993. The MAR fish assemblages were influenced by the surronding substrate. Thus, a group of ubiquitous species, frequent on both substrata and typical of natural rocky habitats (mostly Sparidae) and a group associated with the samples recorded on the Posidonia meadow (mostly Labridae) could be distinguished. Season had only a secondary and slight effect on fish assemblages. The MAR was colonized by 74% of the species in the first two years after deployment. Positive and lineal relationships between species richness, diversity, abundance of necto-benthic species and time since deployment on sandy substrata were found. On Posidonia oceanica deployment time has only increased very sedentary and cryptic species. Although some ways in which the MAR would have a productive function are not rejected, the most obvious MAR working pattern is that of fish aggregation.