The correlation between methanogenic activity and anaerobic reactor performance is intuitive. In this paper, an attempt has been made to suggest a new parameter, defined on the basis of the relative activity of acetoclastic and hydrogen oxidising methanogens, to aid in evaluating the performance and stability of anaerobic reactors. Performance of three bench scale reactors was assessed at different relative populations of the trophic groups of methanogens as estimated through acetoclastic and total methanogenic activity tests. Results indicated that the acetoclastic to total methanogenic activity ratio 0.7 yielded stable and optimal reactor performance. The acetoclastic and total methanogenic activity tests provide a simple technique that may aid in evaluating the performance and stability of anaerobic bioreactors.