A systematic, semiquantitative measurement of intensity of euendolithic microbioerosion aids in understanding the interactions between microorganisms and basalt glass. These interactions are important because they occur widely in ocean basins and lead to incongruent dissolution of glass. No semiquantitative method currently exists for measuring euendolithic microbioerosion. We modify the ichnofabric index (ii of Droser and Bottjer, 1986) to the microendolithic ichnofabric index (MII), a scale-independent, orientation-independent, semiquantitative classification scheme specifically for euendolithic microborings in volcanic glass, but applicable to any medium. Material used to develop the MII comes from the phase one core, Hawaii Scientific Drilling Project #2 well and ranges in age from ~636 to 413 ka. Microtubular features in the glass are linear or curvilinear, ~0.5–2 µm in diameter, 1 µm to >100 µm long, and originate from the margins of glass fragments found in hyaloclastites and pillow lavas. Standard categories of the ii were modified to address the circumstances of the microborings. Percent disruption of primary fabric is the basis of the MII rather than percent disruption of bedding. Six categories are used ranging from no (MII  =  1) to complete disruption (MII  =  6). Modification of the ii extends the measuring scale of bioerosion to near the minimum size range for trace fossils. By extension, MII can be used for any traces that penetrate the associated medium, from microbial borings to dinosaur footprints, if the ratio of measurement length to diameter of the traces is ~30 : 1.