Invariant life-history theory has been used to identify parallels in life histories across diverse taxa. One important invariant life-history model predicts that, given simple assumptions and conditions, size-at-sex-change relative to maximum attainable body size (relative size-at-sex-change, RSSC) will be invariant across populations and species in sequential hermaphrodites. Even if there are broad pecies-wide limits to RSSC, populations could fine-tune RSSC to local conditions and, onsequently, exhibit subtle but important differences in timing of sex change. Previous analyses of the invariant sexchange model have not explicitly considered the potential for meaningful differences in RSSC within the confines of a broader ‘invariance’. Furthermore, these tests differ in their geographical and taxonomic scope, which could account for their conflicting conclusions. We test the model using several populations of three female-first ex-changing Caribbean parrotfish species. We first test for species-wide invariance using traditional log–log regressions and randomisation analyses of population-specific point estimates of RSSC.