A petrographic study of Quaternary (Pleistocene to Recent) calcretes from coastal regions of the western Mediterranean reveals that calcified filaments are abundant and characteristic structures in these terrestrial deposits. From their sizes, shapes and arrangements, these structures are interpreted as calcified organic filaments of soil fungi, algae, actinomycetes and root hairs of vascular land plants. The morphology of calcified filaments depends on: The origin and composition of the organic filament; The state of preservation of the organic filament at the beginning and during its calcification; Whether the mechanisms of calcification are essentially physicochemical, biochemical or a combination of both physicochemical, and biochemical processes. With the exception of two samples, the mineralogy of the calcified filaments is low magnesian calcite. The fabric of this calcite consists of micronsized needles (with long and crystallographic c-axes oriented perpendicular to the organic substrate), rhombs, or plates. The resulting calcified product, formed by mineral encrustation, impregnation or piece-meal replacement, may be a tube or solid rod depending on whether the organic mould of the filament has been left empty or filled with a micritic cement. The recognition of calcified organic filaments in calcretes indicates the former existence of organic activity and, moreover, indicates that these deposits functioned as biological soils.