The female of D. fujiama Fraser has a 2n = 24 karyotype, including a pair of X chromosomes which are similar in size and the largest elements in the complement. The males of both D. fujiama and D. m. moiwanus (Okumura) have the same number of chromosomes, 2n = 23 and n = 12; in each sp. a single X chromosome, which is the largest element, is always observed in the spermatogonial karyotype. At diakinesis and early Ml, the X chromosome consisting of a pair of chromatids is easily distinguished from the autosomal bivalents on account of their different morphology. Preliminary examination of C-banded spermatogonial metaphases and spermatocyte complements at diakinesis in D. fujiama indicates that discrimination between the X chromosome and the autosomes is easy because of a peculiar distribution pattern of constitutive heterochromatin of the X chromosome. Such chromosomal evidence reveals that D. fujiama is characterized by a male heterogametic sex chromosome system of the XO/XX-type. No m-chromosome could be encountered in either sp.