Patterns of ontogenetic variation in the composition of the otoliths of southern Bluefin tuna (SBT), Thunnus maccoyii, strongly suggested that the concentrations f some elements (and particularly strontium) varied episodically, and probably annually. This suggested that counting the numbers of peaks and troughs in strontium concentrations, as measured across the entire growth axis of a vertebra o otolith, could be an accurate and precise way to determine the ages of fully mature SBT. This suggestion was tested by 1) determining experimentally the extent to which otolith composition varies seasonally, 2) comparing ages estimated using otolith composition with those estimated for the same fish by conventional hard-part aging, 3) determining the extent to which different observers examining the same data would agree on the age of a given specimen (which is a measure of the inherent imprecision of the otolith chemical approach), and 4) attempting to validate annual cycles of deposition by measuring changes in the composition of otolith margins for samples collected over an 18 month period.