Cochlear toxicity of tobramycin was compared with that of four other aminoglycosidic anitbiotics. Small groups of guinea pigs received tobramycin (TM), gentamicin (GM), neomycin (NM), or kanamycin (KM) in doses of 50 and 100 mg/kg/day subcutaneously for 1–6 weeks; others received KM 200 and 400 mg/kg/day, or dihydrostreptomycin (DHSM) 400 mg/kg/day. Cochlear microphonics were recorded one week after treatment ended, and hair cell losses were plotted as cytocochleograms. Curves relating maximum response (CMmax) to duration of treatment fell less steeply for TM than for GM and NM, but more steeply than for KM. At 200 and 400 mg/kg KM produced moderate or severe deduction in CMmax after 2 weeks. DHSM 400 mg/kg was without effect in 11 of 12 animals treated for 3–6 weeks. Hair-cell losses were roughly consistent with CM levels. In cochleas with only moderate CM loss, inner hair cells often persisted when outer hair cells had disappeared, but with severe CM loss there was also extensive loss of inner hair cells. [Supported by NIH Research Grant NS-05065 and Program Project Grant NS-05785.]