Survey on a potent toxic cyanobacterium, ~iCylindrospernzopsis~i spp. from freshwater resources, demonstrated the distribution of this organism all over Thailand. There were two distinct morphotypes, straight and coiled trichomes. Two types of terminal heterocyst formation, oval and spearhead shape were also recorded. However, Cylindrospermopsis with straight trichomes with oval shape heterocyst were more commonly. The morphological characteristics of most strains isolated from Thailand and some Japanese stain s were mostly corresponding to the description of ~iC. raciborskii~i, while one strain from Shinobazu-gaike pond of Japan resembling the description of ~iC. philippinensis~i reported earlier. The 16S rDNA sequence including two strains from Australia AF067818 and AF067819 showed high DNA sequence similarity exceeding 99.5%. Therefore, all strains were identified as ~iC. raciborskii~i. The phylogenetic tree of 16S rDNA sequences revealed that all ~iC. raciborskii~i strains formed a defined cluster as a monophyletic group and closely related with Anabaena flos-ague AF317628. Cylindrospermopsin was detected in some strains of ~iC. raciborskii~i. The frequency of heterocyst formation and growth in various cultured conditions were different among the strains of ~iC. raciborskii~i. Heterocyst formation occurred in particular cultured conditions, such as, nitrogen free CT medium, temperature 25 - 30(+,ฐ)C and light intensities 40 - 80 (+,m)mol/m('2)/sec . The growth rate of most strains seemed to relate with the frequency of heterocyst formation. Strains with high growth rates developed larger number of heterocysts. Japanese strains generally formed akinetes better than Thai strains. High light intensity at 200 (+,m)mol/m('2)/sec although inhibited growth, induced carotenoid production of ~iC. raciborskii~i. The PCR fingerprints of STRR sequence by using various primer combinations presented various DNA band patterns and distinguished among the strains of Cylindrospermopsis. Size of amplified PCR product ranged from 0.3 to 5.6 Kb in Thai and Shinobazu-ga-ike strains and 0.2 to 6.7 Kb in Go-no-ike strains. The greatest variation among Thai strains were shown in primer combination STRRIR and STRR3R which were able to obviously differentiate coiled from straight trichomes, even within the cultures isolated from the same population. Furthermore, this pair of primer combination could be distinguished among Shinobazu-ga-ike strains and among Go-no-ike strains. All of the primer combinations showed the DNA band patterns of Thai and Shinobazu-ga-ike strains were highly significant difference from Go-no-ike strains. The temperatures range of growth demonstrated that the Japanese strains grew at a wider range of temperature at 15 to 35(+,ฐ)C, while Thai strains grew weakly at 15(+,ฐ)C with a three days lag phase. Coincidentally one of Thai strain DMKU51014 which formed the same Cluster I-B as Shinobazu-ga-ike strains within phylogenetic tree constructed from the binary data of band patterns different of STRR sequences, had the similar growth characteristic. The observations inferred that Thai and Japanese strains had physiology or evolution adaptation differences. Although they showed ecology differences, all strains, both straight and coiled trichomes had similar pigment composition, fatty acid composition, DNA base composition and heterotrophic growth.