Microbial endoliths occur commonly in tests of planktonic and benthonic foraminifera, as in other types of skeletal and non-skeletal carbonate 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 morphologically characteristic endolith was found only in deep water sediments (between 2,000 and 4,000 m depths). Other modern heterotrophic endoliths appear to be less restricted in their depth distribution. Some endolith species and assemblages of species are persistent through 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 for paleobathymetric indication than either alone.--Modified journal abstract.