Microbial endololiths occur commonly in tests of planktonic and benthic foraminifera, as in other types of skeletal and non-skeletal carbonates substrates in marine sediments. The modern sediments considered in this study were deposited in depths ranging from 200 to more than 4,000 meters. Microbial boring is a strictly benthic activity which afflicts carbonate particles after they have settled at the sediment/water interface. The trace-fossil record of microbial boring is thus a benthic overprint on the carbonate sedimentary record, regardless of its composition. A new morphologicallyc haracteristic endolith was found only in deep water sediments (between 2,000 and 4,000 m depths). Other modem heterotrophic endoliths appear to be less restricted in their depth distribution. There is no apparent correlation of occurrence between particular endolithic and foraminiferal taxa, indicating that the relationship is not a host-parasite one, and that any nutrient requirement by the heterotrophic endolith taxa studied is not specific for the organic product of any particular foraminiferal species. Some endolith species and assemblages of species are persistent throughout geological time (Cretaceous-Recent, Oligocene-Recent). As both microbial endoliths and their host substrates, the benthic foraminifera have independent depth distributions, their co-occurrence provides a basis for a more refined cross-reference or paleobathymetric indication than either alone.