1. Approximately 5,233 acres of tidal marsh on the Blackwater National Wildlife Refuge in Maryland were trapped by Fish and Wildlife Service personnel from 1941-1945, with the closely controlled operations yielding 23,539 muskrats; 13,421 (57 per cent) were males and 10,090 (43 per cent) were females, a sex ratio of 133: 100. This preponderance of males was consistently maintained throughout the entire trapping period each season and is regarded as significant. ...2. In the race discussed, Ondatra z. macrodon, two distinct color phases occur. Of the total catch for the five-year period, 52 per cent were of the black-and-tan phase and 48 per cent brown....3. Males were heavier than females, averaging 2 pounds 4 ounces (1,030 g.) and 2 pounds 2 ounces (962 g.) respectively, or a difference of 2.4 ounces (68 g.). Maximum individual weights were: male-4 pounds (1,814 g.) ; and female 3 pounds 12 ounces (1,701 g.).....4. Carcasses consistently showed a rather small amount of fat, including those coming from luxuriant stands of Scirpus olneyi...5. Average weights of the total catch varied little from year to year....6. Weights increased progressively from January 1 to February 15, followed by a rapid decline....7. Summarization of the catch into bi-weekly and monthly periods showed January to be the most productive month (53 per cent); January 16-31, the most productive bi-weekly period (27 per cent); and February 16-28, the least productive period with only 12 per cent of the average annual catch....8. The best prime condition was generally reached during the last half of February but this period, unfortunately, yielded the smallest number of pelts....9.A brief discussion of pertinent management principles is given.