Gaseous phosphine was found to inhibit biogas formation during the anaerobic fermentation of swine manure. A logarithmic dose-response correlation was obtained with 50% inhibition at a phosphine concentration of approximately 150 ppm. Anaerobic fermentation of acetate and yeast extract measured by biogas formation were not affected at concentrations up to 1000 ppm. Despite the relative tolerance of anaerobic bacteria, phosphine may obviously inhibit the degradation of manure constituents.