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The mass coral bleaching event in South East Asian countries in 2010 caused severe damage to reef dwellers on the coast of the Andaman Sea and the Gulf of Thailand. Bleaching is a process where the symbiosis between corals and zooxanthellae (dinoflagellates of the genus Symbiodinium) breaks down. The symbiosis between Symbiodinium and corals is essential to the productivity, survival and success of corals. Symbiodinium associated with coral hosts is classified into six genetically distinct clades based on nuclear ribosomal DNA and partial chloroplast large subunit rDNA. Each clade possesses unique environmental, ecological and geographic variations, influencing the resilience of corals to elevated temperatures and bleaching. This study was the first field observation of changes in Symbiodinium communities in the Gulf of Thailand over two years including a natural bleaching event from late April to June, 2010. Phylogenetic clades and diversity of Symbiodinium inhabiting soft corals of the genus Sinularia were investigated using the analysis of chloroplast gene sequence variation at the ribosomal large subunit 23S Domain V. A total of 69 Sinularia spp. samples were obtained from two sampling sites off Jorake and Juang Islands (Sattahip, Chonburi Province) during two sampling periods: before (November 2008-January 2010, n??45) and after (November 2010, n??24) the 2010 coral bleaching event. The results revealed a significant change in the Symbiodinium community after the bleaching event. Before bleaching, most of Symbiodinium clade D was detected from Jorake Island while clade C was dominant in the Juang Islands. In contrast, after bleaching, only clade D was identified from both sampling sites. [ABSTRACT FROM AUTHOR]