The early life stages of the diamond squid Thysanoteuthis rhombus from early embryogenesis to post-hatching (0–7 days old) were observed through laboratory incubation using egg masses collected in the southern Sea of Japan. The egg diameter and mantle length increased during embryonic development through hatching. Mantle-length growth was linear over time, and the growth rate was significantly higher at 25°C than at 20°C. The inner yolk was located on the dorsal side of the mantle cavity and increased in volume through hatching. Immediately after they hatched, the hatchlings remained on the bottom of culture plates with their ventral sides facing up, but 2–3 days after hatching they began to swim with their dorsal sides facing up. Feeding experiments were conducted, but none of the hatchlings fed. Statolith growth increments were shown to form daily. Ontogenetic changes that occur from fertilization through post-hatching are discussed.