Apicoplast DNA was first isolated from avian malaria parasite, Plasmodium lophurae, as a 35-kb extrachromosomal DNA fragment [27] and [28], but was initially considered as a mitochondrial DNA (mtDNA). More than 10 years later, other extrachromosomal linear DNA fragments, those around 6 kb in length, were isolated from other Plasmodium spp., and demonstrated to be genuine mtDNA [29] and [30]. Further characterization of the 35-kb DNA molecule showed that it was circular and sequence similarity and phylogenetic analyses suggested that it might be of plastid origin [31], [32] and [33]. Almost complete nucleotide sequences of the 35-kb DNA isolated from P. falciparum [21] and T. gondii (submitted to EMBL, U87145) demonstrated that the genes in the ribosomal operons encoded in the DNA are organized similarly to the plastid genome of red algae. Based on this evidence, the 35-kb DNA fragment was considered to be the plastid genome DNA of apicomplexans, but it was notable that the DNA fragment does not encode any genes related to photosynthesis. The apicomplexan plastid genome is likely to have lost photosynthetic protein-coding genes through adaptation to its parasitic life style during its long evolutionary history. Localization studies of the plastid-like DNA using T. gondii cells identified the multi-membranous structure, which had been known from previous electron microscopy studies, as plastid (apicoplast) of apicomplexans [34] and [35].