Length-frequency histograms of southern bluefin tuna caught for tagging and other purposes by F.V. Estelle Star in the course of survey operations showed heterogeneity in that the catches contained size groups, the differences between which could not be regarded as simply those to be expected between the age groups of a single stock. The times at which these groups appeared in the catches showed that the groups were characterized by distinct distributional behaviour, and this was confirmed by the results of the tagging operations. However, the groups are indistinguishable in their growth rate, so far as this has been shown by the present work, and no evidence is yet available as to the origins or status of these groups.