Babesiaorientalis is an intraerythrocytic protozoan parasite of the buffalo Bubalusbubalistransmitted by the tick Rhipicephalusheamaphysaloides.It is the causative agentof water buffalo babesiosis,one of the most important pathogens of water buffalo incentral and southernChina.As a member of the Phylum Apicomplexa,Babesiaorientalispossesses a relatively independent and alga originated organelleas well,which called the apicoplast.Apicoplasts in other apicomplexa parasitesare involved inthe biosynthesisof fatty acids,isoprenoids,iron-sulphurclusters and haem.Some of these metabolicpathways were shown to beessential for parasite survival,therefore can serve as potentialdrug targets.In this study,we have determined and analyzed the complete genome sequence of B.orientalisapicoplast.It was consist of a 33.2 KbcircularDNA(78.9%A+T).The apicoplast genomeunidirectionally encodes onelargeand onesmallsubunit ribosomal RNAs,24tRNA genes,4DNA-dependent RNA polymerase beta subunits(rpoB,rpoC1,rpoC2a and rpoC2b),17 ribosomal proteins,oneEF-Tu elongation factor,2 Clp protease chaperones,and 14 hypothetical proteins.Meanwhile,it includes two copiesof clpCgene.Furthermore,genome analysis revealed a similar gene structure and organization of theB.orientalisapicoplastto that ofB.bovis.This is the first report of thecomplete nucleotide sequence of B.orientalisapicoplastgenome.