Marginal increment analysis was used to validate the periodicity of opaque increment formation in red throat emperor (Lethrinus miniatus) otoliths and to test for variation in the periodicity and timing of increment formation across age classes, years and regions of the Great Barrier Reef (GBR). Marginal increment analysis demonstrated that opaque increments were deposited on an annual basis, a result supported by a preliminary experiment on a chemically tagged captive fish, although significant regional and temporal variation in the timing of increment formation was observed. Marginal increment analysis revealed a clear annual periodicity in increment formation for all age classes in the southern region but a more ambiguous pattern for most age classes in the northern region of the species distribution on the GBR. Opaque increments appeared to form 1 month earlier in the southern region than in the northern region and, in the southern region, formed 2 months earlier in 1999 than in 2000. No significant variation was observed in the timing of increment formation between age classes. Based on knowledge of the timing of otolith increment formation and spawning season of L. miniatus, an algorithm was developed for the estimation of age in months, irrespective of the time or location of capture.