Growth of muskellunge Esox masquinongy from 12 Ontario sources was investigated by examining 582 samples from the Cleithrum Project archive and other specific studies; 88% of the samples were from angler-caught “trophy” fish. We detail sampling problems and develop methods for resolving them. Muskellunge from some sources were unsexed; sex was discriminated (probability of correct classification, 98.3%) from the von Bertalanffy growth parameters ultimate length (L∞) and growth coefficient, K. When one sex was inadequately sampled, the von Bertalanffy growth parameters of one sex were used to estimate those of the other. When samples were small and inadequate (<11), we used concordance sum of squares to match growth and give an interim estimate from the adequately sampled source with the best growth match. In Ontario populations, mean ultimate total lengths range widely: from 81.4 to 140.0 cm for females and from 70.7 to 115.9 cm for males. Females can be grouped into three types of growth, producing either large-, medium-, or small-bodied fish (ranging from 140 to 127 cm, 126 to 114 cm, and 113 to 102 cm and smaller, respectively). We describe and categorize growth and growth potential to establish standards for detecting change in exploitation and for reviewing minimum size limits (currently underway) based on growth biology to help sustain and even increase the size of muskellunge populations while producing high-quality trophy fisheries.