The Japanese mantis shrimp Oratosquilla oratoria spawned oval eggs, measuring 0.47×0.44mm soon after spawning. Two regression formulae were suggested for the relationship between mean water temperature (X, °C) and the duration of incubation (Y, day), when the animal nursed the disc-shaped egg mass using the thoracopods: Y=58.39-1.85X, Y=111.40/(X-15.26). The hatched larvae developed and metamorphosed to the first juveniles through 11 larval stages over a period of one to two months at 21-28°C. Larval stages and the first juvenile were de-scribed and illustrated in detail. Propelagic larvae in the first and second stages contained yolk and did not feed, but the other nine, pelagic, stages had functional mouth parts and ate a range of sizes of living Artemia cultured in a special medium. Positive phototaxis was observed during the period of the third to the final stages though it diminished as development progressed; larvae in the tenth and eleventh stages often occurred close to the substrate. The first juvenile behaved similarly to the adult. i.e. it lived in an artificial burrow.