Mechanisms of porosity reduction in fine-grained limestones (micrites), the most abundant type of limestone, have been difficult to evaluate because of the fine crystal sizes. Scanning electron microscopy reveals common minute voids termed microfenestrae (∼1.5-40 μm diameter) in Holocene lime muds from all marine carbonate environments studied. Experimental compaction of lime muds greatly reduces abundance of microfenestrae at pressures less than 10 bar. Thus, the abundance of cement-filled microfenestrae in micrites appears to be a widely applicable criterion for recognition of the extent of cementation prior to significant compaction in any given micrite. Microfenestrae in most Phanerozoic micrites studied are about as common as in Holocene lime muds, suggesting that early cementation is a more important mechanism in micrite porosity reduction than recent views have suggested.