Three mosquito-borne human encephalitis viruses (eastern equine encephalitis virus [EEE], St. Louis encephalitis virus [SLE], and La Crosse encephalitis virus [LAC]) are sympatric in the southeastern United States. However, little is known concerning the temporal and spatial pattern of the distribution of these viruses in this area. As part of surveillance activities to detect the transmission of these 3 viruses in the Tennessee Valley area, we developed a single-tube multiplex reverse transcriptase polymerase chain reaction (RT-PCR) assay capable of detecting these 3 mosquito-borne viruses in a single reaction. Three viruses were differentiated by size of amplified products. Sensitivities of the multiplex RT-PCR assay for SLE, EEE, and LAC were 1-3 log median tissue culture infective doses per pool, roughly comparable to the reported sensitivity of PCR detection assays for the individual viruses, and 1 log more sensitive than antigen-capture assays for SLE and EEE. The sensitivity of the multiplex PCR was not changed significantly when carried out in the presence of extracts prepared from 50 uninfected mosquitoes. The cost of the assay is estimated at $2.98 per test, similar to the cost of other RT-PCR-based assays for viruses. However, adaptation of the RT-PCR to a multiplex format adds less than $0.01 to the per-unit cost of an RT-PCR assay targeting a single virus species. Analysis of these data suggests that the single-tube multiplex RT-PCR assay represents a sensitive, specific, cost-effective, and rapid method for monitoring activities of the 3 endemic mosquito-borne human encephalitis viruses in mosquito populations in the southeastern United States.