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Field observations, chemical analysis, electron and light microscopy and radiolabelling experiments provide information on the characteristics, formation and distribution of Fe and Mn rock varnishes in southern Tunisia. Unstable rocks are not surface-coated, but when broken they commonly reveal internal microbial solution fronts. Stable rocks are coated in Fe, and more rarely Mn, rock varnishes. Radiolabelling indicates the presence of many respiring microbes on both Fe and Mn varnishes. Microscopy shows fungi and bacteria, both on the Fe varnishes and within the rock. Only rare, microcolonial fungi occur on Mn varnishes, suggesting that the microbes are within or beneath the varnish. The abundance of microorganisms supports a biogenic origin for rock varnishes. Different microbial communities appear to be involved in Fe and Mn varnish formation. Microbes appear to live beneath the surface of unstable rocks but coat stable rocks which cannot be penetrated, apparently to avoid u.v. light and desiccation. This explains the concentration of elements that form u.v. opaque minerals and clays. The origin of these elements is external to the substrate. These results suggest that rock varnish dating and paleoenvironmental techniques can be applied in southern Tunisia.