Populations increased to 7.8 beds per acre on a managed marsh, with 2.5 muskrats (Ondatra zibethica rivalicia) per bed considered harvestable. Pelt quality was highest in Jan. and the first half of Feb. Three age groups are defined by a new method with subadults those large enough to breed but not mature as to thickness of pelt, length or wt. In total length subadults are 460 to 509 mm., with kitts and mice smaller and adults larger. Pelts vary but large numbers of measurements (from posterior edge of ear to a line between 2 corners of pelt) give accurate age groupings. By this pelt measurement subadults are 197.5 to 222.5 mm. The normal age group ratio is about 8 kitts and mice, 28 subadults and 64 adults per hundred. Breeding is almost continuous, dropping to 8% of adult [female][female] pregnant in Jan. Composite sex ratio for all groups was 59.2 [male][male] to 40.8 [female][female]. Marsh management information is summarized.