This paper deals with cytogenetical and cytochemical studies of the carp (Cyprinus carpio), the funa (Carassius carassius) and their hybrids. When kept under a confined condition, the carp and the funa mate andcan produce hybrids. Reciprocal crosses are also possible with similar results. The hybrids grow regularly with no observed abnormalities in the course of their development. They rank intermediate between the parent species in several characters. The hybrid males are completely sterile, while a hybrid female laid eggs in backcrossing. The spermatogenetic activity in hybrid testes is greatly disturbed. The chromosomes as observed in spermatogonial devision of hybrids are 100 in number, being the total sum of the haploid numbers of the parents, 50 for the carp and 50 for the funa. Meiosis in the hybrid testes is highly disturbed being arrested at early stages of the meiotic prophase. Most of the germ-cells undergo pycnotic degeneration during the period from late leptotene, and no spermatozoa are produced. In some hybrid specimens, the gonads show mosaic structures composed of testicular and ovarian elements, anevidence suggesting that sterility is associated with intersexuality caused by genetic unbalance between the parent species. The DNA amount in spermatogonial nuclei of thehybrids is approximately the same as that of liver nuclei, showing the 2n value. The DNA amount in the pachytene nuclei of the hybrids is less than the 4n value, while the parent species have the reduced amount of DNA in their pachytene nuclei. A consideration was made that the reduced amount of DNA in the hybrid cells may cause the disturbance of cellular activity leading to the subsequent degeneration of cells. Some aspects of enzymatic pattern in the carp, funa and their hybrids are. going on.