M. ehrenbergii was reported from Europe by Focke in 1836. Since then it has been found in Asia, Trinidad, and in 5 states of the U. S. The European form has 10 chromosomes with approximately median centromeres. The 11 individuals of the American form examined have 8 chromosomes. Each pair is easily distinguished from the others. The 2 shortest chromosomes are ca. 14,[mu], long and the 2 longest ca. 27[mu] at mitotic metaphase. These are the largest chromosomes found in American Turbellaria. In the primary spermatocyte 3 bivalents occur and 2 chromosomes, always the same chromosomes, remain unpaired.