Bottom sediments collected from 203 sites on the southeastern continental margin of the United States from South Carolina to central Florida were analyzed for microborings produced by endolithic organisms. Sediment samples were impregnated with plastic and subsequently acid etched to reveal three dimensional models of borings networks. These casts are considered to be diagnostic for specific organisms. All samples were statistically analyzed for relative abundance of individual microborers. Five major microboring types were recognized: (1) filamentous microborings, 1-4 mu m in diameter, of fungal origin, (2) stellate microborings with centric cavities 40-50 mu m in diameter, also of fungal origin, (3) radiate microborings, comprising clusters of tubules, 8-10 mu m in diameter, produced by a septate green alga, (4) tubulate microborings, 15-20 mu m in diameter, produced by a siphonaceous green alga, and (5) spinate microborings, 20-30 mu m in diameter, attributed to sponges. Filamentous fungal borings represent the most abundant endoliths found within the study area. All samples, from the shallowest at two meters to the deepest at 450 m, contained fungal microborings; the relative percentage of grains infested within samples ranged from three to 87%. Stellate fungal borings were more restricted in their distribution, reaching their greatest abundance on the northeastern Florida shelf. Radiate microborings of algal origin were randomly distributed throughout the study area and, where found, generally infested less than five percent of the constituent grains of a sample. The distributional pattern of tubulate microborings of algal origin within sediments of the study area showed a tendency towards linear trends paralleling bathymetric contours. A highly bored inner sediment band is believed to represent a zone of intensive boring related to the present day photic zone; an outer band of sediment containing abundant tubulate microborings is interpreted as relict and marking a "fossil photic zone" associated with a lower sea level stand. Latitudinal variation found within the distributional pattern of this microborer suggests that it may have paleoecological application as a temperate zone indicator. Spinate microborings, although very limited in their distribution and abundance, were restricted to sediments collected from depths in excess of 25 m, suggesting their potential application as a paleobathymetric indicator.