Abstract The alphavirus Sindbis was tested for its ability to mediate cell fusion of tissue cultured invertebrate cells. It was found that Sindbis virus grown in BHK-21 cells produced optimal fusion of BHK-21 cell monolayers after brief exposure to pH 5.3. Fusion of mosquito cells by the same virus occurred optimally at pH 4.6 (a pH which results in the destruction of the virus particle) if the cells were cultured in Eagle MEM. Mosquito cells cultured in insect cell medium could not be fused at any pH tested (as low as 4.0). The differences in the response to pH were not due to an inhibitory component in the media and cells cultured in alternate media required several days to weeks to assume the fusion characteristics of cells maintained in that media. All cells in all media were infected by Sindbis with equal efficiency, as determined by infectious center assay. These data suggest that the pH requirement for initiating the virus mediated fusion event does not reside with the virus alone.