Results are presented of a first-principles calculation of the direct band-gap pressure coefficient a g for a series of Al, Ga and In chalcopyrite semiconductor compounds AgXY 2 (X = Ga, In; Y = S, Se, Te) and the corresponding zinc-blende structure. Good agreement was found between the calculated and experimental pressure coefficients. It was found that a g in chalcopyrites are dramatically reduced relative to zinc-blende compounds, and that the Al → Ga → In substitution lowers a g in chalcopyrites more than in zinc-blende compounds. As a result, the empirical rule suggested for zinc-blende compounds, stating that for a given transition (e.g. Γ 15v → Γ 1c ) a g does not depend on substitutions, has to be modified for chalcopyrites.