The isotopic study (S, O, C) of carbonate fluorapatite (CFA) and pyrite in the Neocomian sequence at Wawal, Polish Lowland, provides a rare occasion to understand factors that controlled fractionation between these phases in the process of bacterial sulfate reduction (BSR). Sulfur isotopic composition of CFA-sulfate is moderately enriched in 3 4 S, but pyrite is highly depleted in 3 4 S with respect to initial Neocomian seawater. We found that the extent of sulfur fractionation between CFA sulfate and sulfur in accompanying framboidal sulfides is larger (~70%%) than can be derived by bacterial activity (~46%%). Isotopic composition of CFA-sulfate (δ 1 8 O(SO 4 2 - )) indicated that SO 4 -H 2 O equilibration could proceed relatively easily in early diagenetic events of Neocomian sediments, due to probable bacterial activity. We speculate how the isotopic data δ 3 4 S(SO 4 2 - ), δ 1 8 O(SO 4 2 - ), δ 1 3 C(CO 3 2 - ) translates into the successive oxidation-reduction processes coupled to CFA and pyrite precipitation.