Parexocoetus mento (Cuvier et Valenciennes) is a small-sized, two-winged flying fish found in the southern waters of Japan ; fig. 1 shows the adult. The writers studied its life history and habits on the western coast of Amakusa Islands in Kyushu. The specimens were collected during the period from May to October, 1955. The results obtained are summarized as follows : (1) The adult appeared in this region from the middle of May to the late September, and the larvae, juvenile and half-grown fish were found from early June to late October. (2) They were attracted to a flood light at night, with the pectoral and ventral fins spread out, and swam sluggishly about near the light, so that they were very easily captured by a dip net. They feed entirely on planktonic organisms, mostly copepods. A great many stomachs have been examined and nothing ever was taken out of them but a pinkish paste of organisms containing many crustacean fragments. (3) The spawning season is from May to September and the peak is in June and July, and the spawning act takes place soon after the sunset and lasts till the next morning. The number of eggs carried by an adult female varied from about 1,000 to 2,000. The egg is spherical, 1.75-1.78 mm in diameters, with about 20 long tendrils distributed over the whole surface. The yolk is transparent and the oil-globule is absent (Fig. 2, 3A). (4) The eggs were stripped and inseminated with milt from the male. The embryonic development was observed and the hatched larvae were reared in the aquarium, fed with the nauplii of brine shrimp. It takes about 10 days at 21, 25℃. water temperature to hatch out, and most larvae hatched out in several hours after sunset. The newly hatched larvae were about 5 mm in total length (Fig. 3 E). (5) The newly hatched larvae swam close to the water surface and darted about actively in the aquarium, and fed actively on the nauplii of brine shrimp. In the larval stage, the body is a little more robust. The fin rays have definitely developed except the pectoral fins. The color of body is dark brownish above and bright yellow brownish below. The ventral fins are dark greenish. (6) In the juvenile stage, the dorsal fin becomes elevated and is wholly black except the basal part. The ventral fins are large and almost brownish. The characteristic three rows of melanophores situated along the dorsal and anal bases and in the lateral portion of posterior body are observed. The barbel of the lower jaw is absent, but the lower jaw is a little produced as in the sauries, Scombresocidae (Fig. 7, 8). These small fishes occassionally made short, sporadic flights and surface skips. (7) In the specimens longer than 25 mm in fork length, the shape of body agrees with the adult form and the color is dark bluish above and silvery white below. They show a tendency to leave this region for the southern waters in winter. (8) For the growth and age-determination, we used the scale characters and the plotting frequency distributions of body size (Fig. 9). In this discussion the scale of adult shows one ring, " cutting-over " (Fig. 10). It is considered that the ring begins to form in February or March and the formation seems to be due to the metabolic change brought about by the production of sexual elements. Sexual maturity is attained at the end of their first year, when the males have reached a size of about 98 mm and the females about 110 mm in fork length. The largest size is about 123 mm in male, about 130 mm in female in fork length. Most fish seems to die after the first spawning. The ratio of males to females was 5 in adult.