Abstract
Najas minor and N. chinensis are newly reported from southern Iriomote Island, which is the southernmost distribution for both species in Japan. The morphology, reproductive phenology, and chromosomal numbers were compared with reports from previous studies. Najas minor on Iriomote Island was larger, up to 120 cm long, had longer leaves (3 to 4 cm long), flowered and fruited year-round and had a new karyotype (2n = 24) for the species in Japan. Najas chinensis on Iriomote Island was within the reported range of morphological and chromosomal variation for the species, but flowering and fruiting were observed almost throughout the years of study. Phenological and morphological descriptions provide new insights into the life cycles and identification of N. minor and N. chinensis.