The possibility of using a mixed community of sulphate-reducing bacteria (SRB) for the biotransformation of phosphogypsum was examined. The greatest reduction of phosphogypsum (g/l) was determined in cultures with lactate (3.3), ethanol or casein (2.7), almost two-fold less in media with glucose or lactose and threefold less in medium with acetate. In media with lactate or ethanol growth inhibition (I = µ (µ_max) was slight (I = 0.80 or 0.79) but much higher in the case of cultures with lactose (I=0.41), glucose (I=0.54), acetate or casein (I=0.62). In those cultures in which the concentration of sulphides was very high (about 600 mg HS^-/l) and the concentration of acetic acid did not exceed 10 mg/l (e. g. in cultures with glucose), inhibition of SRB was mainly caused by H₂S. In cultures with transient low pH value (e. g. containing lactose or acetate) the factor causing stronger growth inhibition was acetic acid. A condition for obtaining high SRB activity in media with phosphogypsum and fermentable carbon sources is constant monitoring of the reaction of the medium and/or counteracting the accumulation of toxic concentrations of hydrogen sulphide and acetic acid.