We demonstrate a simple and efficient method for separating metallic from semiconducting single-walled carbon nanotubes (SWNTs) using density-gradient ultracentrifugation. Density differences between metallic and semiconducting SWNTs, which enable SWNT separation by electronic type, are created using a single surfactant, i.e., sodium dodecyl sulfate (SDS), rather than a complex mixtures of surfactant, as is used in current separation schemes. SDS strongly adsorbs onto the surface of metallic SWNTs over semiconducting SWNTs by the mirror-charge phenomenon. Therefore, metallic SWNT-SDS assemblies have relatively smaller buoyant densities than semiconducting SWNT-SDS assemblies; thus, the metallic assemblies are easily collected at the most buoyant top fractions, whereas the semiconducting assemblies are collected at the bottom fractions. We also demonstrate that this protocol is valid regardless of the SWNT production method; that is, SWNTs grown by high-pressure carbon monoxide conversion (HiPco) and the arc discharge method. Optical absorption shows that the heavy bottom fractions consist of highly pure semiconducting nanotubes, whereas the buoyant top fractions consist of highly pure metallic nanotubes. In addition, films made of the separated metallic SWNTs exhibit lower sheet resistances than unsorted SWNTs by 53% for arc discharged and 64% for HiPco SWNTs, as expected.