Standardization of agar diffusion disk tests for antibiotic susceptibility testing with anaerobes is complicated by the marked variability in the growth rates of different strains and by the need for a complex, enriched medium for testing nutritionally fastidious strains. Furthermore, the composition of the agar medium and the anaerobic environment itself will affect the activities of some drugs. To assess the accuracy of currently available disk methods, 64 clinical isolates were tested by four different disk technics. The anaerobic method of Wilkins and associates and that of Sutter and co-workers were both evaluated. In addition, the standard Bauer-Kirby method and that of Barry and colleagues were modified slightly to permit testing of anaerobes; both proved to be unsatisfactory for anaerobic tests. In most respects, the method of Sutter and co-workers was superior to the other disk methods studied, although an inordinantly large number of tests gave zones that fell into the intermediate category. None of the disk methods was entirely satisfactory for testing nutritionally fastidious strains.