Visual scuba diving censuces used to assess the effects of fishing prohibition on abundance and size structure of littoral fish populations by comparing the same benthinc communities inside and outside the protected area of Medes Islands (NE Catalonia, Spain). The total number of species found was 43 in the reserve and 44 outside, but the mean value of species richness per sampling station was significantly higher in the protected area. However, H' here diversity, heavily affected by the presence or absence of large schools of pelagic species, showed no significant differences between sites. The prohibition of fishing for 6 years is the first factor affecting the quantitative and the qualitative structure of fish populations ("reserve effect"), and depth is the second factor. Thus, except in the cases of Serranus cabrilla and Mullus surmuletus, all other vulnerable species are highly sensitive to the protection measures. The siez stricture of all vulnerable species was found to be absolutely different at the reserve sites than in the unprotected zones, an the modal size classes os size frequency distributions were always higher in the reserve than outside. The reserve effect was significantly responsible of the differences observed in this change on size structure. Some highly vulnerable species, such as Epinephelus guaza and Sciena umbra, have only been found in the protected areas Others, such as Sparus aurata, Diplodus cervinus and Dicentrarchus labrax, were much frequent inside the reserve.