The intestinal contents of adults of 12 of the 18 species of parasitic lampreys were examined microscopically and tested for blood to determine whether a species fed mainly on blood, flesh or blood and flesh. The diets of each species are shown to be related to characteristics of their dentition, buccal glands and velar tentacles. The trends exhibited by those relationships were used to hypothesize as to the diets of those six species for which there were anatomical but not dietary data. The dentition aids the suctorial disc in attaching the lamprey to its host and removing host material, while velar tentacles prevent material entering the branchial cavity and buccal glands produce lamphredin that has anticoagulant and lytic properties. In blood feeders, such as Petromyzon marinus and Mordacia species, the w-shaped transverse lingual lamina and hook-shaped longitudinal laminae bear numerous, fine cusps, which are ideal for rasping a hole in the host. In contrast, in flesh feeders, such as Lampetra fluviatilis and Geotria australis, the transverse lingual lamina is u shaped and the longitudinal laminae are straighter and possess at least one stout cusp, thereby facilitating the removal of host flesh through gouging. The buccal glands are generally larger in blood feeders than flesh feeders, presumably reflecting a need to produce anticoagulant continuously. Each velar tentacle contains a central cartilaginous rod, surrounded by a space which, during feeding, becomes engorged with blood and thus more rigid. They are small (≤1 mm long) and few (2–6) in blood feeders and large (typically ≥2 mm long) and numerous (3–42) in flesh feeders, which, in particular, require a mechanism for preventing solid material entering the branchial pouches and thus potentially clogging the gills. On the basis of recent cladistic analysis, blood feeding is ancestral to flesh feeding in Northern Hemisphere lampreys (Petromyzontidae).