The selectivity of four hook sizes (STELL brand, Quality 2335, numbers 12, 9, 6 and 4) used in a semi-pelagic longline was studied in the Azores. Two species were caught in sufficient numbers for modelling of selectivity: the black spot sea bream (Pagellus bogaraveo) and the bluemouth rockfish (Helicolenus dactylopterus dactylopterus). A maximum likelihood method was used to fit a versatile model which can be used to describe a wide range of selectivity curves; from bell-shaped to asymptotic. Significant differences in size selectivity between hooks were found for both species. In the case of Pagellus bogaraveo, the smallest hook (number twelve) had the lowest catch rates and all hooks were characterised by logistic-type selectivity curves, with sizes at 50% selectivity of : 27.9, 30.4, and 32.8 cm for hooks numbers twelve, nine and six. The number nine hook was the most efficient for Helicolenus d. dactylopterus, with selectivity curves varying from strongly skewed to the right for the number twelve hook to logistic-type for the number six and the number four hooks. Sizes at 50% selectivity for this species were 16.8, 18.7, 20.7, and 22.0 cm respectively.