Ontogenetic change in body density and shape of phyllosoma larvae was examined for the Japanese spiny lobster Panulirus japonicus (von Siebold, 1824) using laboratory-cultured specimens from hatching to the final phyllosoma stage before metamorphosing to the puerulus stage. Phyllosomas have a unique body form that is leaf-like and dorso-ventrally compressed with several appendages, and a long planktotrophic life in the ocean. Ontogenetic change was not found in the larval body density, and the mean density (1.097 g cm^-3) was greater than the seawater density (1.024 g cm^-3), indicating that they must employ strategies to control their buoyancy. The body shape of the phyllosomas changed during development into the puerulus stage where pleopods on the pleon and the pereiopods function as swimming organs and walking legs, respectively. The pleon rapidly developed after the mid-stage VI phyllosoma (10.6 mm body length (BL)). The thoracic pereiopods, including the third maxilliped, that function as swimming organs during the phyllosoma stage, generally showed negative allometric growth, indicating that the potential swimming ability decreased with development. Aspect ratios, defined as (long-axis length)² /(area), of the cephalic shield and the thorax increased from stage I(1.5 mm BL) and tended to be stable from stage V (5.9 mm BL). Relatively reduced swimming ability might be recovered with the aid of the cephalic shield and the thorax, which have higher aspect ratios that may generate a larger lift force with less power.