The effect of natural mineral on the mono-digestion of maize straw was evaluated in continuously stirred tank reactors (CSTRs) at 38 A degrees C. Different strategies of mineral addition were studied. The organic loading rate (OLR) was varied from 0.5 to 2.5 g volatile solid (VS) L-1 d(-1). A daily addition of 1 g mineral L-1 in reactor 2 (R2) diminished the methane production by about 11 % with respect to the initial phase. However, after a gradual addition of mineral, an average methane yield of 257 NmL CH4 g VS-1 was reached and the methane production was enhanced by 30 % with regard to R1. An increase in the frequency of mineral addition did not enhance the methane production. The archaeal community was more sensitive to the mineral than the bacterial population whose similarity stayed high between R1 and R2. Significant difference in methane yield was found for both reactors throughout the operation.