The effect of pH on growth rates and sulphide toxicity thresholds of a variety of key bacterial groups involved in anaerobic digestion is described. At pH 7.0-7.5, the growth rates of methane-producing bacteria (MPB) and sulphate-reducing bacteria (SRB) were similar. Above and below this pH range, MPB and SRB, respectively, have more favourable growth properties. Inhibition of all the bacterial groups studied was related to the total sulphide concentration in the pH range 7.2-8.5. Propionate-utilising SRB were the most sensitive of the bacterial groups, both in pure culture and in sludge samples, to high concentrations of total sulphide. At lower pH levels (6.8-7.2), the levels of sulphide which resulted in 50% inhibition of the growth of the bacterial groups were similar, although values obtained were higher for sludge samples than for pure cultures.