Antarctic toothfish (Dissostichus mawsoni) from the Ross Sea (CCAMLR sub-area 88.1) have been aged assuming that one translucent zone is formed annually in the otoliths of this species. However, no evidence to validate this assumption has previously been presented. In the current work, sagittal otoliths were examined from D. mawsoni that had been injected with oxytetracycline some years before recapture. The number of zones between the fluorescent oxytetracycline line and the otolith margin indicates that one translucent zone is formed annually in otoliths of post-mature D. mawsoni. Otoliths from four distinct juvenile length-frequency modes were also examined, and the translucent zones counted. Zone counts were consistent within modes, and increased by one in each consecutive mode. This indicates that the modes represented year classes and that one translucent zone is formed annually in otoliths of juvenile D. mawsoni. Mean fish lengths at ages 0.5, 1.5, 2.5, and 3.5 years were estimated to be 14, 28, 37, and 46 cm TL, respectively. These estimates agree with growth curves calculated previously from a sample of data that was large but lacked any fish younger than 3.5 years. Counting translucent zones in the otoliths of D. mawsoni appears to be a valid method for determining the age of this species.