Abstract: The diversity and abundance of cyclopoid copepods were investigated at four stations along the hydrothermal vent at Kueishantao Island, north-eastern Taiwan. Zooplankton samples were collected by surface tows. Qualitatively, 21 species of cyclopoids belonging to five genera and three families were observed. Our analysis revealed a number of abnormal protrusions (TLAs) on cyclopoid copepods, which were more frequent on the dorsal body surface and in members of the family Corycaeidae. The number of TLAs was highest at the hydrothermal vent plume (HVP) zone and among adult copepods. The potentially high levels of toxic minerals erupting from the hydrothermal vents as well as low pH and high temperatures may weaken the copepods' exoskeletons, thereby increasing susceptibility to infections, indicated by the HVP-related increase of tumoural processes. It is suggested that some of these abnormalities are related to the occurrence of symbiotic tantulocarids, a topic yet to be studied in this interesting environment.