The blackmouth catshark Galeus melastomus Rafinesque, 1810 is commonly caught all along the southern Tyrrhenian Sea. The knowledge about the biology of this species is limited, especially in this area. Data collected from five trawl surveys carried out during spring 1999, 2000 and 2001 and autumn 2000 and 2001, are analysed in order to fill the gap of information about population structure, length-weight relationship, and reproductive biology of the blackmouth catshark in the studied area. On the whole, 1,046 specimens (537 males and 509 females) were caught between 300 and 700 m depth. The percentage of positive hauls (55%) is observed in the deepest strata (300-700 m). The length of the specimens ranged from 10 to 50 cm TL both for males and females. Young specimens represented the bulk of the catch. The length frequency distributions were plurimodal. However, some significant differences were observed in population composition according to depth and season. On the basis of weight-length relationship an isometric growth was observed.