Two series of the oriental river prawn, Macrobrachiurn nipponense (DE HAAN), females of which were both collected from a river in Fukuyama City, Hiroshima Prefecture, have been reared in the laboratory in order to observe their mating and spawning behaviours. About two days before the mating the male begins to approach and to follow the female. A few hours before the mating the male holds the female between the widely extended second pair of pereiopods and actively touches the female with his antennae and first pair of pereiopods. The mating depends largely upon the pre-mating molt of the female and takes place night and day, immediately after the pre-mating molt. The male turns the soft-shelled female to a ventral side position and then presses down from above for 2-3 seconds. Soon after this, the male separates from the female. This behaviour usually is repeated 2-4 times after a single pre-mating molt. The female lays eggs within 6 hours after mating and carries on the pleopods 11-12 days. The spawning activity begins around 7p.m. and continues for one or one and a half hours. The female raises up the abdomen unto an angle of 45°- 90°against substratum and vibrates vigorously all pairs of pleopods to release the larvae just hatched for a few seconds. Then she returns to the normal posture and stops vibrating the pleopods. This activity is repeated many times during the spawning period at a interval from 20-30 seconds to a few minutes, though almost all larvae hatch out the first 30-45 minutes. A single female lays eggs twice and more in one season. The interval between the release of the larvae and the succeeding egg-laying is one or two days.