Publisher Summary This chapter focuses on the evaluation of criteria for the recognition of Diphyllobothrium species. Reliable data about intraspecific variations are needed before proper assessment the relative stability of size or any other taxonomic character in classification. Not only are data required on the kinds of variants and the frequency with which they occur but to properly evaluate them it should be determined if the characters involved are hereditary or nonhereditary. The dimensional and morphic characters of internal and external characters are probably sufficiently stable to serve as specific diagnostic characters if the material being compared is from the same specific final host of comparable age, sex, and host diet, harboring an equal number of worms of the same postinfection age. While the basis for identification at the species level, regardless of the category definition, is being sought within the context of the anatomical concept, even when the limits of variations have been established, all available criteria should be taken into account.