The purpose of this report is to review some of the potential effects of acidic precipitation on the microbiota, and also to discuss contributions to this problem by microorganisms. Since all life in our biosphere is ultimately dependent on microbial activities, the dearth of supportive data on microbial processes related to acid rain is a serious shortcoming. Although not directly applicable, some information can be gleaned from studies on environments simulating the effects of acidic precipitation (e.g. acid mine waters, coal ash effluents and in vitro experiments). Another indirect source of information is the literature on the major groups of microorganisms involved: that is, those responsible for the cycling of sulphur, nitrogen and carbon, primarily in aquatic and terrestrial ecosystems. For the purpose of clarity, as there is much overlap, this review will be organized into three main sections on the three cycles followed by a section on cycle interactions. The discussion of the reactions and the bacteria involved in the three cycles is not intended as an exhaustive review of the latest research in these areas as this would not be within the scope of this report. It is intended only as a guide to focus on bacterial and chemical processes which may be involved in environments affected by acid rain.