An extrachromosomal nucleic acid element was detected in high-molecular-weight DNA preparations form Babesia equi merozoites. This extrachromosomal element was shown to be DNA rather than RNA and had an apparent fragment size of about 9 kilobase-pairs (kb). Hybridization experiments using purified 9-kb DNA as a probe revealed sequence homologies with extrachromosomal DNA from two other Babesia species.