A series of field and laboratory experiments were conducted at Kyushu in 1978-81 to investigate grain shedding in japonica-indica hybrid rice bred in S. Korea. Breaking tensile strength ranged from 84 to 188 g in the hybrid cv. and from 195 to 245 g in japonica type cv. Most of the hybrid cv. had easily shed grain compared with Japanese cv. In the hybrid cv., breaking tensile strength decreased from 1 wk after heading to a min. after 3 wk, then remained constant until harvest whereas in japonica cv., breaking strength did not vary after heading. Hybrid cv. had an abscission layer with parenchymatous cells which had almost completely cracked at 3 wk after heading, similar to their indica cv. parents. The breaking strength of hybrid cv. was correlated with the thickness of the supporting zone and was lower in yr of cool weather, particularly at the booting stage. Shedding was also increased when there was frequent rain during ripening. Cellulase activity was observed in the pedicel of plants with an abscission layer, whether or not the abscission layer cracked, but not in pedicels of plants without an abscission layer.