The secondary aerenchyma formation in hypocotyl just below the soil surface was investigated by using the young seedings of wild soybean and six summer leguminous crops grown under upland and excessive moisture conditions for 14 days. Under the upland conditions, secondary aerenchyma was scarcely observed in any species. Under the excessive moisture conditions, however, there was an interspecific difference in the secondary aerenchyma formation. The area of secondary aerenchyma per transverse section of hypocotyl under the excessive moisture conditions was largest in soybean 'Asoaogari' (9.77mm2), followed by soybean 'Akisengoku' (7.80mm2), wild soybean 'D5' (4.37mm2), cowpea 'Sanjyakusasage' (2.63mm2), mung bean 'Bundomame' (1.97mm2) and mung bean 'Acc. 7703' (1.52mm2), those in the other crops were less than 1 mm2. The ratio of secondary aerenchyma area to stele area was largest in wild soybean, followed by soybeans, suggesting that the ability of secondary aerenchyma formation in soybean was almost equivalent to that in wild soybean. Since a significant positive correlation at the 1% level was found between dry weight ratio (the ratio of dry weight of the aerial part under the excessive moisture conditions to that under the upland conditions) and the area of secondary aerenchyma under the excessive moisture conditions (r=0.738), it is suggested that the ability of secondary aerenchyma formation is related to the wet endurance in leguminous crops.