The genetic relationship among Chaetoceros curvisetus, Ch. debilis and Ch. gracilis is studied using mol. biol. methods.The nuclear-encoded rDNA gene and ITS sequences (about 2800 bp corresponding to 3'-end of SSU rDNA gene and the 5'-end of LSU rDNA gene) were amplified, cloned and sequenced.The sequences homol. anal., phylogenetic tree construction, and AP-PCR fingerprint comparison were conducted.Results show that sequence homologies between each part of rDNA gene are 97%-100%, and ITS sequences have more than 99% homologies.The evolutionary distances among them are very close, but AP-PCR fingerprint patterns of the 3 Chaetoceros are different.The close genetic relationship of the 3 Chaetoceros is concluded, and that 3 Chaetoceros belong to one family is suggested.