Methanogenesis is the most important anaerobic biodegradation process in nature, which is accomplished by three different kinds of bacteria - hydrolytic, acetogenic and methanogenic bacteria (MB). An experiment was performed to determine the rate-limiting step of methanogenesis under the influence of various phosphine concentrations (100, 300, 500, 700 and 1000 ppm). It was found that the growth of fermentative bacteria (FB) was severely affected by higher concentrations of phosphine (700 and 971 ppm), while the growth of hydrogen-producing acetogenic bacteria (HPAB) and MB was not affected severely at higher phosphine concentrations. Thus, HPAB and MB are less sensitive to phosphine compared with FB, which means that hydrolysis, and fermentation step is the rate-limiting step during methanogenesis under the influence of phosphine. It is recommended that special attention be paid to the first stage of methanogenesis under high concentrations of phosphine during anaerobic wastewater treatment.