© 2020, Japanese Society for Plant Systematics. All rights reserved. Ludwigia grandiflora (Michx.) Greuter & Burdet is an invasive aquatic plant native to an area ranging from the southeastern United States to South America. In previous reports, L. grandiflora in Lake Biwa has been identified as L. grandiflora subsp. grandiflora, a hexaploid subspecies (2n = 48) or L. grandiflora sensu lato. Species of Ludwigia sect. Jussiaea, are morphologically very similar. Morphological characters, especially floral characters and cytology (chromosome number) have been used to identify species and subspecies. In our study, the morphological characteristics of L. grandiflora in Lake Biwa substantially corresponded to those of L. grandiflora subsp. hexapetala (Hook. & Arn.) G. L. Nesom & Kartesz, a decaploid (2n = 80) subspecies. The chromosome number of L. grandiflora in Lake Biwa was 2n = ca. 80, the same as in L. grandiflora subsp. hexapetala (2n = 80). Importantly, L. grandiflora in Lake Biwa is not L. grandiflora subsp. grandiflora, but rather L. grandiflora subsp. hexapetala. This re-iden-tification has implications for management in Lake Biwa, since the life history of L. grandiflora subsp. hexapetala has been studied in Europe.