Electrochemical studies and weight loss measurements on the biocorrosion of mild steel in the presence and absence of sulphate reducing bacteria (SRB) grown either in lactate/sulphate and or in lactate/nitrate aqueous media, have been performed. Tafel analysis of the quasi-steady state polarisation curves gave the corrosion current densities, j(corr), of 20 and 30 mu A cm(-2) for mild steel samples in the SRB culture in lactate/sulphate and lactate/nitrate media, respectively. In the absence of SRB the corresponding values of j(corr) were 19 and 9 mu A cm(-2), respectively. Corrosion current densities given by the linear polarisation resistance method were very close to the ones given by Tafel analysis. A comparison of the corrosion rates obtained either from electrochemical or from weight loss data is given for all the media under study. Open circuit potential and redox potentials measured over two days showed great differences for the lactate/sulphate and lactate/nitrate growth media. Scanning electron microscopy (SEM) observations resulted in good correlation with open circuit potential data.