Maghemite can transform to magnetite by anaerobic bacteria through solid route. This transformation has important significance both to mineralogy and environmental magnetism. This study investigates the interactions between sulfate-reducing bacteria (SRB) and maghemite, with special emphasis on the effect of bacterial activity on mineral transformation rate. Two types of experiments were designed: one with sulfate and the other without sulfate. After incubating for 31 days, in experiments with SO₄ ^2- and SRB, about 16.7% sulfate was reduced to acid volatile sulfides. Some Fe(II) that was derived from the reduction of maghemite reacted with AVS to form monosulfides, bisulfides, and polysulfides. The decrease in particle sizes of maghemite was associated with the release of Fe(II). In experiments without sulfate, Fe(II) reduced by SRB remained in the mineral and no secondary precipitates was observed. X-ray diffraction and Mössbauer spectroscopy analyses showed that maghemite transformed gradually to magnetite, at a greater rate in experiments with sulfate, which suggests that sulfate promoted both the activity and the electron transfer ability of SRB. Under natural and artificial anaerobic conditions, sulfate could be an important factor that constrains the transformation of maghemite to magnetite.