Thai rhiostomids were collected and classified. Six species were identified i.e. Rhiostoma asiphon Moelendorff, 1893; R. chupingense Tomlin, 1938; R. housei (Haines, 1855); R. jalorensis Sykes, 1903; R. samuiense Tomlin, 1931, and R. hainesi Pfeiffer, 1862. There were 9 distinct unidentified morphotypes. The dominant shell character of type 1 is proboscis shape with zigzag pattern on dorsal side, and long tube present. Type 2 is sub-discoidal shape with zigzag pattern on dorsal side, and short tube present. Type 3 is sub-discoidal shape, both sides present zigzag pattern. Type 4 is sub-discoidal shape with brown in dorsal and white in ventral .Type 5 is proboscis shape, disconnected part turn down, and tube absent. Type 6 is proboscis shape, sharp triangular projection and sharp behind apertural projection keel present. Type 7 is sub-discoidal shape with monotonous color form. Type 8 is sub-discoidal shape, dorsal side presents zigzag pattern, and tube absent. Type 9 is proboscis shape, long slender projection present. Shell morphometric analysis of 8 characters by ANOVA Duncan’s multiple range test showed the significant difference among 6 species and 9 morphotypes, R. asiphon differs from the rhiostomids by shell morphology, operculum, radula and genitalia, which I proposed to re-classify to Pterocyclus asiphon (Moellendorff, 1893) by similarity of operculum shape and number of cusps on first marginal teeth. Shell, radula, and operculum morphology, anatomy of genital organ were used for cladistic analysis using Hennig86. R. housei performs plesiomorphic, which related to its big ranges of distribution in the north, north east, and central part of Thailand , while R. Chupingense and R. jalorensis distribute in the south, R. hainesi is the eastern species. P. asiphon is close related to an outgroup Cyclophorus volvulus. Rhiostoma shows allopatric speciation which has no overlapping zone.
