Karyotypes of Hydra oligactis Pallas, 1766 (2n = 30, FN = 60) and H. circumcincta Schultze, 1914 (2n = 30, FN = 56) were studied in mitosis and meiosis, using routine Giemsa-staining and C- and Ag-bandings. It was shown that in H. oligactis, C-blocks are situated in centromere regions of all chromosomes. In H. circumcincta, NORs were found to be present in one of the larger chromosome pairs. The species have no morphologically distinguished sex chromosomes. They differ from each other in chromosome morphology, showing 30 M/SM/ST in the former and 26M/SM + 4A in the latter.