PIP: HIV-1 genetic subtypes B and E are currently circulating in Thailand. Subtype E accounts for more than 90% of heterosexual transmission nationwide, while subtype B is transmitted mainly among IV drug users. This paper reports an HIV-1 Thai E isolate which yielded discordant results in serotyping and genotyping assays. An HIV-1-infected mother enrolled in Bangkok in a perinatal transmission study was identified independently as subtype B by V3 serotype by St. Mary's and HIV/AIDS Collaboration laboratories using different in-house assays. Her plasma was also screened for other HIV-1-specific immune responses, yielding a pattern of antibody reactivity similar to other subtype E sera. The genetic subtype of the isolate was identified by heteroduplex mobility assay (HMA) to further characterize it. Analysis results suggest that the woman was infected with HIV-1 subtype E. The env region encoding C2-V3 was subsequently sequenced to clarify the discordant results between the V3 serotype B and the genetic subtype E.