Out of 78 diseased guppies imported from Singapore to Japan, 43 fish were found infected with ciliates. Thus, pathological examination of guppies naturally infected with ciliates and identification of a ciliate sampled from guppies in Singapore were conducted. Most of the ciliates parasitized the inside of scales, muscles or internal organs, while some invaded into the eye socket and spinal cord. No damage was found in the intestinal epithelium. From these findings, it is considered that the main invasion route of these ciliates was from the scales of hosts to internal organs through muscles. As the results of morphological examination using silver impregnation, the examined ciliate was identified as Tetrahymena corlissi Thompson, 1955, which has been reported as a pathogen of guppies in North America.