The morphology of 3 species of Lernaeenicus: Lernaeenicus affixus, Lernaeenicus longiventris, and Lernaeenicus radiatus, mesoparasitic copepods of fishes from the Atlantic coast of North America, is documented. There was significant structural variability in the cephalothorax and holdfast horn complex on the anterior embedded end between individuals in all 3 species. The functional morphology of holdfast structures of the parasites was influenced by the requirements for adequate anchoring in host tissues and was related to the types of tissues and host species infected: attachment directly to bone by individual L. affixus produced lamellae and short, unbranched holdfasts; the hammer-shaped cephalothorax provided with short holdfasts exhibited by members of L. longiventris was found in hard, dense musculature; and elongate, dendritic holdfasts of specimens of L. radiatus were found in soft muscle. Based on morphological characteristics, L. longiventris, L. affixus, and L. radiatus are considered as separate and distinct species even though the latter 2 are similar in size and appendage structure and infect the same species and individual fish hosts. All 3 species are described from specimens in the collection of the United States National Museum.