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The identification of Casuarina grown in Taiwan is difficult probably because of introgressive hybridization. In this study, 456 individual samples were evaluated for RAPD variation in order to assess the genetic diversity and to identify hybridization in Casuarina in Taiwan. Eleven primers were used and 81 polymorphic bands were scored. The average Nei's gene diversity of Casuarina grown in Taiwan (0.198) was significantly higher (P < 0.001) than those of native provenances of C. equisetifolia (0.140), C. cunninghamiana (0.148), and C. glauca (0.106) based on Duncan's new multiple range test. The results of cluster analysis and principal coordinates analysis support the hypothesis of hybridization of Casuarina in Taiwan. A dendrogram based on RAPD data showed that most sampling sites formed a subgroup that linked with native provenances of C. equisetifolia, indicating that most plants in Taiwan are close to but not typical C. equisetifolia. Two sampling sites in Taiwan linked with native provenances of C. glauca, indicating that some plants were close to this species. The two- and three-dimensional diagrams derived from principal coordinates analyses indicated that the plants were located among C. equisetifolia, C. cunninghamiana, and C. glauca and generally closer to the first species. These results strongly indicate that most plants of Casuarina currently grown in Taiwan are the result of introgressive hybridization involving C. equisetifolia, C. glauca, and perhaps C. cunninghamiana. The results also imply that most plants are derived from the backcrossing to C. equisetifolia. [ABSTRACT FROM AUTHOR]