The structures within the gill pore of lampreys are shown to be a new taxonomic character. Papillae along the posterior margin of the gill pore cup, termed marginal papillae, vary in number, size, shape, and arrangement among species. The size and shape of a central process that extends posteriorly from the mid-lateral area of the wall of the gill pore cup is also of taxonomic significance. Examples are provided to show that there is stability of the character within an individual and among populations of a particular species. However, an example of the variation in counts over the range of Lampetra ayresii indicates that it is the average number of papillae, their arrangement, shape, and associated size in addition to the shape and size of the central process that may be of taxonomic significance. Examples of the usefulness of the new characters are provided for L. pacifica and Entosphenus folletti. The numbers and size of the marginal papillae readily distinguish L. pacifica from L. richardsoni. The number of marginal papillae and shape of the central process indicate that the holotype and paratypes of E. folletti may not be the same species. The full taxonomic value of these structures remains to be determined, but these initial studies indicate that the structures within the gill pore cup provide a method of improving the interpretation of speciation in lampreys, particularly for nonparasitic species.