Structure of the mouthparts and foregut of Panulirus ornatus phyllosomata (stages I-VI hatchery reared, IX-X wild caught) were examined using scanning electron microscopy and histology to gain an understanding of ingestive and digestive processing mechanisms, to identify potential shifts in diet during development, and to suggest appropriate physical characteristics for the development of a suitable formulated diet for commercial aquaculture of this species such as size, texture, and buoyancy. Mouthpart and foregut structure indicates that P. ornatus phyllosomata are capable of ingesting zooplankton of any hardness during this life history stage and are only limited by their ability to capture and manipulate prey as the mandibular molars are well developed to masticate prey further. Mouthpart morphology changes little during development, however, the disposition of the mouthparts and size of the mouth aperture increases with each successive stage of development, suggesting a greater capacity to manipulate and ingest larger prey. ne foregut of all developmental stages consists of a single chamber, with well-developed grooves, channels and setae, but lacks a gastric mill. Presence of well-developed main brushes, lateral setae, and development of a functional filter press at stage IV suggests an increased ability to triturate and filter prey internally, reducing both the time spent externally manipulating prey with the mouthparts and the vulnerability to predation in the open ocean. The results presented here suggest that formulated diets larger than > 428 mu m, with a firm/hard consistency that allow the dactyli of the second and third maxillipeds to penetrate the diet without causing fouling of the setae would be suitable.