The chromosome morphology and number in somatic and germinal mitoses were studied in some species of Macrobiotus. Most of these species (M. areolatus, M. coronifer, M. harmsworthi, M. hufelandii and bisexual biotype of M. richtersi) have 2n = 12 chromosomes, but in two acquatic species with similar morphology (M. dispar and M. pullari) 2n = 10 chromosomes are found. The chromosome number in males and females of M. areolatus and M. richtersi is the same. In the ripening oocytes and in the first blastomeres the metaphasic chromosomes show a peculiar large size. No idiograms were made because the chromosomes are too small, similar and without detectable centromeres.