Lethrinus nematacanthus Meeker is widely distributed from southern Japan to tropical Pacific. The spawning season seems to extend from early June to early August in western coasts of Kyushu. The author operated artificial insemination of this fish on June 8, 27 and 30, 1954, at Tamanoura, Goto Islands, Nagasaki Prefecture, and observed the egg development and hatched larvae (Plate 14, figs. 1-14). The egg is bouyant, colorless and spherical in shape, measuring 0.78-0.83 mm in diameter, with a single colorless oil-globule measuring 0.16-0.19 mm in diameter which is transformed into elliptical shape, measuring 0.15-0.16 x 0.19-0.20 mm, during development (Figs. 5-10). The hatching took place in 39 hours at the water temperature 19.2-21.5° C, and in 27 hours at 23.0-24.2℃. The larva just hatched is 1.60-1.80 mm in total length with the oil-globule situated in the anterior part of the yolk (rarely in the middle of yolk), partly protruding from the yolk surface. The myotome number is 9+17=26 (vertebral number of this fish is 10+14=24) (Fig. 10). In 2.5 days after hatched the yolk was entirely consumed and the anus advanced forward to under 4th myotome (Fig. 14).