Moonmilk represents a conspicuous but controversially discussed precipitate of cave settings. Here, new electron backscatter diffraction microscopic and petrographic evidence on the origin of moonmilk calcite is presented. Calcite fibres in a moonmilk mat from the walls of an active cave (Tunnel-Cave) in Devonian massive limestones in the northern part of the Rhenish Massif (Germany) show orientations of the crystallographic c-axis independent of the morphological fibre orientation. This observation and the morphology of the fibres are in agreement with microbially induced, as opposed to abiogenic, calcite precipitation. Carbon-isotope data are higher (1·9 to 3·3‰) than those commonly measured regionally in speleothem calcite (stalagmites, stalactites and flowstones), an observation attributed to kinetic effects. In combination, these findings add independent evidence to the complex interplay of inorganic and bio-induced carbonate precipitation in cave environments.