Blastocystis sp is probably the most common intestinal protozoan of humans. This taxon is known to include more than 17 subtypes, some of which likely cause human disease. We investigated the distribution of Blastocystis subtypes in Thai patients admitted for a variety of conditions at a hospital in northeastern Thailand. Fresh fecal samples, positive for Blastocystis by microscopy, were individually cultured in Jones' medium (n = 20) and each sample was used for amplification and sequencing a fragment of 18S rDNA. BLAST search and phylogenetic analysis demonstrated that Blastocystis subtypes ST1 (20%), ST3 (60%), ST6 (10%) and ST7 (10%) were present. No clear link between gastro-intestinal symptoms and any particular subtype of Blastocystis was apparent. Thus, there is a need to extend the work to evaluate clinical signs and subtypes in a larger cohort of patients.