Acephalic cysticercus (Ac), a rarely developed multilobulated and nonencysted form of larvalTaenia, causes hydrocephalus or adhesive arachnoiditis in the ventricles and subarachnoidal space that often lead to fatal outcome in affected patients. Ac has been proposed to originate fromT. soliumon the basis of morphological features, while no molecular data supporting the presumption have been available. In the present study, we investigated the immunological properties as well as molecular characteristics of Ac that was obtained surgically from 6 patients. Immunoblotting of the cyst fluid from Ac samples demonstrated the constitutive expression of aT. soliummetacestode (TsM) 10 kDa protein. Specific antibodies against the truncated 10 kDa protein, which appears to be species specific for TsM cysticercosis, were detected in both serum and cerebrospinal fluid samples of Ac patients. Nucleotide sequences of mitochondrial cytochromecoxidase subunit I (COI) and NADH dehydrogenase subunit 1 (ND1) genes of Ac were almost identical to those ofT. soliumbut differed substantially from those of the otherTaeniaspecies. In phylogenetic analysis, Ac clustered withT. soliumin a well-supported clade. Our results strongly suggest that Ac may have originated fromT. solium.