The effect of zinc on the biotransformation of phosphogypsum, COD reduction and growth rate (µ_max day^-1) of an SRB community and Desulfotomaculum ruminis in media with sodium lactate or ethanol was examined. Depending on the form of zinc (Zn₃(PO₄)₂ x 4H₂O, ZnSO₄ x7H₂O, ZnCl₂, Zn(NO₃)₂ x 6H₂O) and its initial concentration (0-80 mg Zn²⁺/l) lower sulphate reduction and COD reduction was observed. The effect of Zn²⁺ also depended on the composition of the studied populations and carbon source in the medium. The lowest inhibition of specific growth rate was determined in cultures of the pure strain and in medium with zinc phosphate (with lactate or ethanol IC₅₀=63 or 75 mg Zn²⁺/l, respectively) and the highest in cultures of sulphate-reducing bacterial communities in medium with zinc nitrate (with lactate or ethanol IC₅₀= 35 or 20 mg Zn²⁺/l, respectively).