Long-chain fatty acids (LCFA) can inhibit methane production by methanogenic archaea. The effect of oleate and palmitate on pure cultures of Methanosaeta concilii and Methanosarcina mazei was assessed by comparing methane 11% and 64% methanogenic inhibition respectively. This study indicates that M. production rates from acetate before and after LCFA addition. For both methanogens, a sharp decrease in methane production (> 50%) was observed at 0.5 mmol L-1 oleate, and no methane was formed at concentrations higher than 2 mmol L-1 oleate. Palmitate was less inhibitory than oleate, and M. concilii was more tolerant to palmitate than M. mazei, with 2 mmol L-1 palmitate causing concilii and M. mazei tolerate LCFA concentrations similar to those previously described for hydrogenotrophic methanogens. In particular, the robustness of M. concilii might contribute to the observed prevalence of Methanosaeta species in anaerobic bioreactors used to treat LCFA-rich wastewater.