This study aimed at comparative karyotype analysis and measurement of the nuclear DNA amount in giant miscanthus, Miscanthus x giganteus, and its hypothetical ancestors: eulalia grass (M. sacchariflorus) and porcupine grass (M. sinensis). The triploid chromosome number 2n = 57 in M. x giganteus and the diploid chromosome number 2n = 38 in the other species were confirmed. In the karyotype of giant miscanthus three satellite chromosomes were observed, whereas in M. sinensis there were two and in M. sacchariflorus probably four chromosomes of this type. In the first species 1-4 B-chromosomes were evidenced. The highest proportion of the C-banding/DAPI (4',6-diamidino-2-phenylindole) positive heterochromatin was found in M. sinensis cv. Gracillimus, and the least in the M. sinensis M07 genome. The banding patterns observed in M. x giganteus resembled those observed in M. sacchariflorus. The 2C nuclear DNA content in M. x giganteus was 7.47 pg and in M. sacchariflorus it was 5.14 pg; two analysed lines of M. sinensis differed in 2C DNA value (5.18 pg and 5.49 pg).