The Galapagos Sailfin grouper, Mycteroperca olfax, locally known as bacalao and listed as vulnerable by IUCN, is culturally, economically, and ecologically important to the Galapagos archipelago and its people. It is regionally endemic to the Eastern Tropical Pacific, and, while an important fishery resource that has shown substantial declines in recent years, to date no effective management regulations are in place to ensure the sustainability of its Galapagos fishery. Previous estimates of longevity and size of maturity for bacalao are inconsistent with estimates for congeners, which questions the accuracy of prior estimates. We set out to rigorously assess the age, growth, and reproductive biology of bacalao in order to provide more accurate life history information to inform more effective fisheries management for this species. The oldest fish in our sample was 24 years old, which is 2-3 times greater than previously reported estimates of longevity. Parameter estimates for the von Bertalanffy growth function (k= 0.07, L∞= 119.1 cm TL, and to= -2.33 years) show bacalao to grow much slower and obtain substantially larger asymptotic maximum length than previous studies. Mean size at maturity (as female) was estimated at 61.1 cm TL, corresponding to an age of 7.8 years. As a protogynous hermaphrodite, size at sex change (from adult female to adult male) occurred at 83.7 cm TL, corresponding to 15.2 years of age. We found that sex ratios were extremely female biased (0.015 M:1F), with a large majority of the individuals in our experimental catch being immature (73%). Our results show that bacalao grow slower, live longer, and mature at a much larger size and greater age than previously thought, with very few mature males in the population. These findings have key implications for the fishery of this valuable species and urge for a long-due species management plan to ensure its long-term sustainability.