The effects of application of the herbicides fluazifop-p-butyl and fomesafen and the commercial mixture of these herbicides on the microbial activity of a soil, cultivated with common bean under no-till (NTS) and conventional-till (CTS) systems, were evaluated. Microbial respiration was monitored for 63 days after application (DAA) of the herbicides, and the following evaluated at 12 and 51 DAA: microbial biomass carbon (MBC), microbial quotient (qMIC), metabolic quotient (qCO2), percentage of bean root colonisation by mycorrhizal fungi and grain yield at the end of the cycle. A greater microbial respiratory rate was observed under NTS, with fluazifop-p-butyl providing the lowest respiration. At 12 DAA, MBC and qMIC were most affected negatively by fomesafen and by the commercial mixture of the two herbicides. Mycorrhizal colonisation was affected by the herbicides only at 12 DAA under CTS; however, in both periods, the highest value was found under NTS. All the herbicides caused a decrease in the MBC and qMIC values at 51 DAA; the qCO2, which is related to the soil system stability, indicated a greater NTS balance over CTS. The herbicide fomesafen induced lower stability in the system. Lower grain yield was obtained without weed control (no herbicides) and with fomesafen-only treatments, which may be attributed to the high weed infestation in the experimental area.