Abstract: Balantidiosis is a cosmopolitan parasitic zoonosis. In the study， a Balantidium coli was isolated from the cecum of a severely diarrheal pig in Zhejiang province of China. RPMI1640 medium was used to cultivate the trophozoites of B.coli. Trophozoites grew well in the RPMI1640 medium at 28℃ or 37℃ and the maximum growth density of 818×103 and 231×104cells·mL-1 was gained on the fifth and fourth day post the inoculation， respectively. Cytostome， cytopyge and food vacuole were obviously observed under the light microscopy in the trophozoites. Macronucleus obviously showed dumbbell\|like after DAPI staining. Two different types of trophozoites， large one and small one， existed in B.coli. And two different sequences of 18s rRNA gene were gained for the B.coli after the PCR and DNA sequencing. Divergence between the two sequences was 06%. The two sequences formed a single branch separated from the reference 18s rRNA gene sequences of B.coli in the phylogenetic tree. In addition， a lot of microorganisms were seen at high magnification of microscopy in the B.coli. Species for these microorganisms were unknown. Three novel sequences of 16s rRNA gene was gained after the PCR with B.coli culture as the template. Relationship between the three novel sequences and the endocytobionts was needed to be further confirmed.

Key words: Balantidium coli, in vitro cultivation, 18s rRNA gene