Carbonate substrates prepared from conch shells and inorganic cleaved calcite were planted both at and below the sediment-water interface in a back barrier sound of North Carolina, within a mudbank of Florida Bay, and along a barrier reef transect in Belize. SEM examination of plastic casts of microboring networks formed the primary basis of study, supplemented by light microscopy of isolated endolithic organisms and TEM examination of doubly embedded material. A diverse assemblage of endolithic forms was detected in substrates planted at the sediment-water interface. In contrast, a less diverse and distinctly different assemblage of endolithic forms was found within substrates planted as much as 160 cm below the sediment-water interface. This is the first known report of such activity within buried marine sediments. The most abundant subsurface endoliths were two coccoid forms, separated on the basis of size, surficial texture, and morphology. Each type had several growth variants believed to be stages of binary, cellular fission. Other endolithic forms detected include a filamentous, irregular, polygonal network, an irregular, flattened mass, and a regular, crenulate, flattened disc. TEM examination of one of the coccoid forms suggests a procaryotic blue-green algal or bacterial origin. The affinities of the other forms are unknown, but resemble endolithic traces attributed to fungi, bacteria, and Actinomycetes. The regular discoids and irregular flattened masses are found only in association with the filamentous form and may be reproductive bodies. Subsurface endoliths appear to be restricted to finer-grained, reducing sediments, and may be utilizing the more abundant interstitial nutrient supply as well as, or instead of, organic matrices within the mollusk substrates. Endolithic activity within buried marine carbonate sediments has important consequences in that it broadens the environmental conditions under which microborings may form, suggests that endolithic heterotrophs may significantly affect the surrounding microenvironment within sediments, and may result in porosity development during early sediment burial.