We investigated body mass variation during the reproductive season in male and female Scopoli's shearwaters (Calonectris diomedea) breeding in Linosa island, Sicily channel. Birds were weighted in three main periods: i.e. before and during egg-laying (mid May - mid June), around hatching (second half of July) and during the early stages of chick rearing (mid July - mid August). In addition, some measurements were also taken around fledging (second half of October). When possible, the same individuals were weighed repeatedly. Overall, there was a marked between-sex difference in body mass, males weighing on average c.100 g more than females. Body mass was higher in males before the engagement on breeding activities (just after the return from their winter migration) and decreased soon thereafter. Conversely, body mass was lower in females during laying and increased during the initial phase of incubation. No other significant variations on the time course of body mass emerged from the analyses. The between-sex difference in body mass profile could be related to a differential effort during the early phases of the reproductive season.