Our continuous investigations revealed that a coastal herb, Lysimachia mauritiana, exhibited a wide-ranging intraspecific karyotypic polymorphism (18 cytotypes) in the Ryukyu Archipelago of Japan. Recently it is suggested that the small island Takarajima including 11 cytotypes showed the highest intraspecific karyotypic diversity both within and among populations in the Ryukyus. To explore karyotypic variations on its adjacent three islands, Amamioshima, Kakeromajima and Tokunoshima, karyomorphological and cytogeographical analyses were conducted in 452 individuals from 27 localities throughout the islands. A total of five different chromosome numbers (2n=16, 17, 18, 19, 20) and 15 cytotypes were recognized in the area. Except for two, two to six cytotypes coexisted in every locality. The islands showed different dominant cytotypes, i.e. 16 (6m) in Amamioshima Is., unknown in Kakeromajima Is. and 18 (6m) in Tokunoshima Is. Furthermore, it is suggested that cytotypes in northeast area in Amamioshima Is. might be converged to 16 (6m) in the near future by dynamic analyses in cytotype.