Fetal counts of muskrats (Ondatra zibethicus) yielded an average of 2.97 ± 1.06 fetuses per litter (range: 1 to 5). Placental scars averaged 7.97 ± 4.34 per breeding female (range: 1 to 22) and indicated production of 2.7 litters per breeding female per year. Corpora lutea counts disclosed that litter sizes ranged from I to 7, averaged 3.54 ± 1. 15, and differed significantly from fetal counts. Litter size as determined by lodge surveys (i = 2.18 ± 0.25) was less than that determined by fetal counts and represented post-partum mortality. Mortality estimates indicated a loss of 16.1% between ovulation and fetal counts; an 18.5% loss occurred between the time fetal counts were made and the time that muskrats reached the 1- to 5-day age class. Subsequent losses of 7.4%,5.4%,9.9%, and 9.4% occurred between 1-5, 6-10, 11-15, 16-20, and 21-24 day old age classes, respectively.
