The morphogenesis of the gut during nauplius stages of Lernaea cyprinacea was reported to occur, without apparent morphogenetic movements, by a non-simultaneous and spatially determined differentiation of the various regions. A developing foregut was first seen at the nauplius-II stage, immediately after moult; on the other hand, the midgut did not appear until the moult to nauplius III. Gut development initially involved some cells containing refractive granules in their cytoplasm. At the nauplius-III stage, strongly Pyronin-positive cells without granules appeared among those with granules. Subsequently, the mouth parts and the hindgut were formed. Only in the late nauplius III and early copepodid-I stages did all parts fuse into a continuous digestive tube. The finding that the entire gut apparatus develops during the naupliar life was taken to suggest that this stage is part of the embryonic development of L. cyprinacea.