Thirty months after the construction of the Loano artificial reef a study of fish abundance was started. The method used both direct censuses at monthly intervals and limited fishing activity with longlines and trammels. The monthly census was carried out by two divers in the central part of the artificial reef made up of pyramids, each consisting of five concrete cubic blocks with 2-m sides. The census was carried out in two main phases: first the observer counted the fish moving around him for 15 min and then he explored the holes and the internal corridors. The second phase consisted of a 25-m crosswise movement between two pyramids. Observations were carried out over a period of 2 years, from February 1989 to February 1991, and 66 species of fish and invertebrates of fishing importance were caught and/or observed in the Loano artificial reef; 41 species (of which three were Crustaceans and two Cephalopods) were censused during the above-mentioned dives. The most numerous families were Sparidae and Labridae, followed by Serranidae. The predominance of the first two families seems to be one of the general characteristics of rocky sea-bed fish communities in the northwest Mediterranean. Seasonal changes in the qualitative and quantitative assemblages of fish are described. An important contribution to these changes was the appearance of juveniles, mainly in the latter half of the summer and in the autumn. It was in this period that the maximum fish density occurred, while the minimum was registered in June. The number of fishes increased with the age of the artificial reef and the evolution of macrobenthos settlement. In particular the development of algae improved the reef's attractiveness to the Labridae species.