Monochoria C. Presl is aquatic flowering plants belonging to Pontederiaceae. It consists of 8 species worldwide, occurring in tropical and subtropical regions. Currently, taxonomic status of Thai Monochoria remains unclear due to its morphological plasticity and different classification viewpoints. Some authors accept three monotypic species i.e. M. hastata, M. elata and M. vaginalis, whereas others recognized infraspecific taxa. To clarify this problem, intensive specimen collections were made throughout Thai floristic regions. Then, 22 quantitative and 23 qualitative characters from 550 specimens were analyzed. Cluster analysis indicated that 550 specimens could be divided into five groups. Similarly, canonical discriminant analysis showed that it comprises of five groups distinguished by lamina width of floral-leaf, petiole length of floral-leaf and flowering stem length. To recapitulate, phenetic data support the segregation into five groups: M. vaginalis and its closely related species, M. elata and its closely related taxon and M. hastata, a monotypic species.
