Hydrilla japonica Miquel, Ann. Mus. Bot. Lugduno-Batavi 2: 271. 1866; Blyxa angustipetala (Ridley) Masamune var. laevissima (Hayata) Masamune; B. laevissima Hayata.

Stems elongated, 10-20 cm tall, branched. Leaves cauline, lanceolate, 3-6 cm × 1-3 mm, veins 3, apex acuminate. Spathes sessile, green, 1-3 cm × 1-3 mm. Flowers bisexual. Sepals green with purple midvein, linear-lanceolate, 2-4 × 0.5-1 mm. Petals 6-10 × 0.5-1 mm. Stamens 3; filaments 1-3 mm, smooth; anthers 1.8-2.5 mm. Ovary conic; styles 3-4 mm. Fruit 1-2.5 cm. Seeds 30-60, narrowly elliptic, smooth. Fl. and fr. May-Oct. 2n = 72.

Paddy fields, ponds, channels. Anhui, Fujian, Guangdong, Guangxi, Guizhou, Hainan, Hubei, Hunan, Jiangsu, Jiangxi, Liaoning, Sichuan, Taiwan, Zhejiang [Bangladesh, India, Japan, Korea, Malaysia, Myanmar, Nepal, New Guinea, Thailand, Vietnam; Europe].