Blastocystis spp. is commonly found in the feces of humans worldwide. Infection has been reported as asymptomatic, acute symptomatic, and chronic symptomatic. This wide range of responses to infection could be related to the genetic diversity of morphologically indistinguishable specimens obtained from infected hosts. The former name Blastocystis hominis is now reported as Blastocystis spp. because of its genetic diversity. Blastocystis is recognized as a complex of subtypes that have not been fully characterized as independent species. The finding of Blastocystis spp. in feces from several animal species suggests a zoonotic potential. Based on conserved regions of published nucleotide SSU rDNA sequences from all Blastocystis subtypes found in GenBank, a PCR and sequencing protocol was developed. The ~500 bp SSU rDNA gene fragment amplified by this PCR is highly sensitive compared with published primers and contains highly variable regions that allow phylogenetic analysis of Blastocystis. These primers were used to detect and subtype Blastocystis spp. specimens from naturally infected humans, primates, cattle, pigs, and chickens. Based on these findings, application of this method can elucidate the complexity of this heterogeneous genus and its role in human and animal disease, as well as its zoonotic potential.