The decapodid stage of Lebbeus comanthi Hayashi and Okuno and the first zoea of Thor amboinensis (De Man) are described based on laboratory-hatched eggs from females collected from Japan. The decapodid stage of L. comanthi is readily distinguished from that of L. groenlandicus by the carapace without anteroventral denticle and tooth behind rostrum, the absence of the antennal spine, the four-segmented outer flagellum of the antennule, the absence of the palp of the mandible, and the telson with posterior margin con caved medially and without dorsolateral spine. The carapace with anteroventral denticle and the third abdominal somite distinctly curved in lateral view distinguish the first zoea of T. amboinensis from that of T. dobkini Chace and T. floridanus Kingsley. Larval characters of the genus Thor are summarized.