Scanning microscope investigations were carried out on mandibles, labrum and paragnaths of several species of Mysidacea and Euphausiacea. Gut content analyses were in agreement with morphological observations. It appears that the morphology of the peri-oral structures and especially of the mandibles reflect the feeding habits, and that the well known relationships between the size of the molar process and the incisor one, i.e. a large molar process is associated with herbivorous feeding, is not always verified. Bacescomysis abyssalis and Bentheuphausia amblyops are believed to be mostly saprophagous, Boreomysis inermis and Meganyctiphanes norvegica are mostly phytophagous, Hemimysis speluncola is omnivorous, while Siriella armata and Thysanopoda orientalis are carnivorous species.