The effect of an electric field on the photoluminescence of single-walled carbon nanotubes was investigated. Individual SWNTs embedded in polymeric film were deposited on microscope slides with electrodes. The fluorescence intensity and spectra of single semiconducting SWNTs were acquired. When SWNTs in polymer were subjected to electric fields of up to 10 7 V/m, a drastic decrease in their fluorescence intensity was observed. The effect was reversible and reproducible. SWNT fluorescence intensity was well approximated by inverse hyperbolic cosine of the electric field, with a single quenching parameter k. It was shown that the effect was induced by the electric field parallel to SWNT axis while the perpendicular component did not produce detectable quenching. The quenching process was found to be enhanced for long nanotubes. Bulk sample fluorescence studies indicated that the electric field quenching also becomes stronger with the decrease in bandgap energy. Potential theoretical models will be discussed.