Data on the relationship between the total length and weight of the muskellunge, Esox m. masquinongy, based on measurements of 264 fish caught by anglers from Lake St. Clair are presented. The fish ranged in length from 30 to 54 inches and in weight from 6.5 to 37 pounds. The plotting of weights of individual fish against their respective lengths indicated a definite relationship between those measurements. Agreement was close between the data on muskellunge from Lake St. Clair and muskellunge (Esox m. ohiensis) from Chautauqua Lake, New York. Similar data on muskellunge (probably some Esox m. immaculatus and E. m. masquinongy) from scattered inland lakes of Michigan indicated that those fish may be slightly heavier than individuals from Lake St. Clair.