To provide an age-length key for use in modelling the age structure of the Patagonian toothfish stock around South Georgia, we used otoliths to estimate the age of a sample of 264 Patagonian toothfish (Dissostichus eleginoides) captured in the longline fishery. We used a thin section grinding machine to reveal a transverse plane through the otolith nucleus and between crenellations. A single reader undertook three readings of the sections, estimating age using criteria corresponding to those given by the CCAMLR Otolith Network for toothfish from South Georgia. Age estimates during the first two readings were significantly biased relative to reference ages but the third reading, after the reader had read more than 3000 otoliths, was not biased. For data pooled from this study and the South Georgia fisheries survey in 2000, estimates of the Von Bertalanffy parameters were L8 = 123.8, K = 0.10 and t0 = -2.1 for males; for females, L8 = 144.9, K = 0.085 and t0 = -2.0. Using these age data, we estimated total mortality to be Z = 0.054.