Abstract
The chromosome number of the muskrat (Ondatra zibethica L.) was determined to be 54 in an investigation of the mitotic and meiotic chromosomes. All chromosomes are one-armed with the exception of one small pair which is biarmed. Further, the X chromosome, one of the largest of the complement, seems to have a small secondary arm making possible an identification of the pair. A systematic investigation of the chromosomes, and especially the sex chromosomes at different stages of the spcrmatocytogenesis, has been performed.