Dugesia dorotocephala (Woodworth) is a common planarian of the United States, widely distributed in springs and spring-fed marshes; in most of the populations the individuals are devoid of a reproductive system and multiply by fission; in some localities, however, sexual specimens are abundant. The Author has illustrated the chromosome set of asexual and sexual races, on material kindly supplied by Dr. Marie M. Jenkins of the Madison College of Harrisonburg (Virginia). In all populations so far studied the complement is of 16 chromosomes. In the sexual race meiosis is normal both in female and in male line. The oocytes show 8 bivalents, which differ in size and in chiasma frequency. In fertilized oocytes the sperm head undergoes a normal transformation in male pronucleus. Relationships of dorotocephala with the European species of Dugesia are discussed.