Sediments collected from 63 sites on the Puerto Rican shelf and slope at depths ranging from intertidal to 530 m were analyzed for associated microborings. Scanning electron microscope examination of three dimensional plastic casts of microboring networks form the primary basis for this study. In all, 27 microboring forms are distinguished on the basis of size and shape of boring casts, mode of branching, overall boring pattern and associated accessory structures. Although most of the microborings exhibit wide bathymetric ranges, many of the forms tend to occur more abundantly within restricted depth zones, suggesting a three-fold subdivision. The upper photic zone assemblage (intertidal to 20 m) is characterized by the endolithic blue-green algae Hyella gigas, Hyella caespitosa, Hyella tenuoir, Hormathonema sp., and an unidentified endolith likely of algal origin (form D). The lower photic zone assemblage (20 m - 85 m) is characterized by the green algae Ostreobium quekettii and Codiolum polyrhizum , the Conchocelis -stage of an endolithic red alga, and a problematic algal form (A). The aphotic zone is characterized by the absence of algal borings and the presence of fungal form Il and a tubular form of uncertain affinity. Microborings in the upper photic zone are typically oriented perpendicular to substrate surfaces; those of the lower photic and aphotic zones parallel substrate surfaces and penetrate only slightly into substrate interiors. Microborings as trace fossils appear to have high paleoecologic potential inasmuch as they are believed to be biologically specific and often are morphologically distinctive. Microborings have been reported in sediments ranging in age form Cambrian to Holocene with many ancient endoliths resembling extant forms. Differences in direction of penetration between photic and aphotic endolithic organisms may be sufficient criteria for differentiating paleophotic zones in ancient carbonate sequences. A Pleistocene example is presented from the Fort Thompson Formation of south Florida which indicates that impregnation techniques utilized in endolithic analysis of modern sediments may find application in studies of ancient microboring assemblages.