Carbonate components of relict continental margin sediments off North and South Carolina show abundant evidence of microborings related to endolithic fungi and algae. Microborings were observed in bottom sediments collected from 165 sites ranging in depth from the intertidal to 780 meters. Forty-two of these samples were fixed in buffered formaldehyde to preserve endolithic organisms which were subsequently isolated for study through decalcification of skeletal substrates. Plastic impregnated thin sections and etched slabs, the latter revealing three dimensional models of boring networks, were also used to distinguish three main types of microborings: a) fungi, 1-4 microns in diameter, b) a septate green alga, 10 microns in diameter, and c) a diagnostic siphonaceous green alga, 15-20 microns in diameter. Of these endolithic plants, fungi were the most abundant as well as most widespread. A distinctive, siphonaceous green alga defined three linear bands of highly bored sediments believed to reflect nearshore zones of high algal boring activity. The innermost zone, extending from the present day intertidal to depths of approximately 25 meters, is considered to be presently active; the outer two zones on the shelf are interpreted as marking relict shoreline positions associated with lower sea level stands. In situ organic erosion by microboring organisms is believed to be an important source of carbonate fines in Carolina shelf sediments. Molluscan components are being selectively attacked and removed from the carbonate fraction through the activity of endolithic fungi and algae. The relative abundance of carbonates in Carolina shelf sediments is therefore believed to be not only related to skeletal productivity and degree of dilution by clastics, but also to the degree of organic erosion to which sediments have been subjected.