The origin of lime mud in ancient shallow-marine carbonate platforms is one of the most vexing mysteries in sedimentary geology. Possible sources previously suggested include the physical and biological disintegration of animal and algal bioclasts, mineralizing pelagic organisms, and spontaneous whitings from either abiotic precipitation or calcification of suspended picoplankton and organic matter. The styles of occurrence of micrite- walled tubes referred to Girvanella in lower Paleozoic limestones argue that occasional permineralization of mucilaginous sheaths of the benthic filamentous cyanobacteria precursor could have produced lime mud when the tubes fell apart. Estimated volumes are comparable to those calculated for the modern Bahama platform. The copious growth of cyanobacteria typical in shallow-marine settings indicates that this mechanism could account for a considerable amount of lime mud, especially in the early Paleozoic.