The influence of phosphogypsum (a waste product of phosphoric acid production), on the activity of Desulfotomaculum ruminis bacteria in a lactate medium was tested. In order to establish the optimum conditions of the reduction of sulphates present in the phosphogypsum, the influence of the concentrations of phosphogypsum, organic substrate and inoculum in the medium on the degree of conversion was studied. It has been shown that phosphogypsum present in the medium at a concentration of up to 20 g/dm3 is not toxic towards the bacteria and can be used as a source of assimilative sulphates. In the optimum conditions of the process (the concentrations of phosphogypsum 10g/dm3, lactate 20.32 g/dm3, inoculum 4%v/v) the conversion of sulphates to sulphides after 7 days reaches over 92%.