The genus Mesocestoides was created in 1863 by Vaillant to receive a new species of cestode found by him in Viverra genetta and named M. ambiguus. Unfortunately, the description of this form is such that, although the generic characters can be recognised, it is impossible to say to which of the several species found in mammals it should be referred, and therefore Fuhrmann (1908) regarded it as a species inquirenda. Weiss (1910), in redescribing a form of Mesocestoides from the same host, gives no details which would enable the parasite to be distinguished from the other members of the genus. He notes that his form is only 50 mm. long, while that described by Vaillant was 160 to 240 mm. long. However, as Neumann (1896) points out, M. lineatus has a size varying between 30 and 250 mm. The size of M. ambiguus therefore is only of use in showing that it does not belong to the species M. bassarisci or M. cæstus referred to later in this paper. It still leaves its identity a matter of speculation as, in addition to M. lineatus, M. mesorchis has dimensions which fall within these limits.