The relative growth of Inimicus japonicus larvae and juveniles was examined for reared specimens. Inflexion points in the relative growth of body parts expressed by allometric formulae were concentrated at two body sizes, 5.5-6.1 and 8.1-8.2 mm in body length (BL), at which developmental and behavioral events mark stages in the early life history of I. japonicus. At 5.5-6.1mm BL, the larvae were in the flexion stage. At 8.1-8.2 mm BL, the postflexion larvae metamorphosed into juveniles and started to change from a pelagic to a benthic habitat. The abrupt growth of upper and lower jaw lengths until 8 mm BL and body size from hatching to the commencement of feeding enables I. japonicus larvae to feed on Artemia nauplii from earlier stage and smaller size than other species. The abrupt growth of pectoral fins until 6.7-8.2 mm BL is considered to enable I. japonicus larvae to swim using large and developed pectoral fins. The large change in the relative growth of pectoral fins at 6.7-13.5 mm BL corresponded to the period of metamorphosis into juveniles and the change of habitat from pelagic to benthic.