Abstract The study of sulfur isotopes of the sulfate within the carbonate-fluorapatite lattice and of some gypsum, associated with phosphorites in two important phosphorite provinces, the Late Cretaceous-Eocene Mediterranean belt and the Permian Phosphoria Formation, shows that the origin of the sulfate within the carbonate-fluorapatite is marine, the δ34S for these samples usually being equal to or higher than the corresponding seawater values. The unusually high values obtained for many samples suggest that at least these phosphorites were formed in an early diagenetic reducing environment during bacterial sulfate reduction. The low δ34S values of the gypsum associated with the phosphorites show that at least part of the gypsum originates from oxidized biogenic sulfide (pyrite), thus supporting the former hypothesis.