Sea urchin populations were monitored in the Medes Islands Marine Reserve (NW Mediterranean) and an adjacent unprotected area in order to (1) describe temporal variability in abundance and population size-structure of sea urchins within each area (from 1991 to 1997), and (2) to compare these areas to investigate the role of fish predation level in determining sea urchin population structure over time (from 1995 to 1997). Abundance of the sea urchin Paracentrotus lividus (Lamarck) was monitored at 7 sites (4 inside and 3 outside the reserve), and 2 distinct habitats (fields of big boulders and vertical walls). P. lividus exhibited signficant variability in density over time on boulders, both inslde and outside the reserve, whereas its density dld not vary significantly on vertical walls. P hvjdus populations differed significantly in size-structure across sites, varying from unimodal to blmodal among sltes and years. Density and mean size of P. lividus were not significantly different between the protected and the unprotected area in either of the 2 habitats. Abundance of the sea urchin Arbaca lixula (L) was monitored from 1995 to 1997 on vertical walls and boulders (4 sites inside and 3 sites outside the reserve). Density of A. lixula differed slgnlficantly over time on boulders, but it did not on walls. On vertlcal walls, density and mean slze of A. lixula were not significantly different between areas in both habitats. This study shows that in the Medes Islands region, sea urchins exhibit striking short-term fluctuations in abundance, which can lead to misinterpretation of larger-scale temporal patterns. The comparison between the protected and the unprotected area does not support the hypothesis of fish predation as the most. important factor affecting P. lividus populations in the Medes Islands, as patterns of lower sea urchin density relative to the unprotected area nearby were not maintained over time. These facts indicate that factors other than fish predation are very important in determining sea urchin population structure in the northwestern Mediterranean.