The addition of exogenous NH4Cl to poultry manure and synthetic medium was used to study the effect of ammonia-nitrogen on the activity and composition of a methanogenic consortium. Results indicated that the production of biogas and methane was not affected by the variation in NH4Cl concentration within the range 2-10 g dm(-3) (0.5-2.6 g N-NH4 dm(-3)). At higher values of ammonium (10-30 g dm(-3) or 2-8 g N-NH4 dm(-3)) a significant decline in both parameters (by 50-60% for biogas and 80-90% for methane) was observed. A significant decrease in the numbers of bacteria of all physiological groups (especially proteolytic and methanogenic) was observed when more than 30 g NH4Cl dm(-3) (7.8 g N-NH4 dm(-3)) was added to the fermentation medium. The addition of 10% (w/v) of powdered phosphorite ore enhanced the production of biogas and methane at NH4Cl concentrations up to 30 g dm(-3), and also changed the composition of the methanogenic consortium. A partial recovery in the numbers of proteolytic and methanogenic bacteria coupled with the decrease in the density of sulphate-reducers was observed. High concentrations (more than 50 g dm(-3)) of NH4Cl seemed to cause irreversible inhibition of methanogenesis which could not be eliminated by the addition of phosphorites.