Single-walled carbon nanotube/polyethyleneimine (SWNT/PEI) composite fibers are fabricated by coagulation-based spinning of SWNTs dispersed in aqueous solution by various surfactants. The spun SWNT/PEI composite fibers are either solid or hollow (see Figure), and have useful mechanical properties and over a hundred times the electrical conductivity of supertough SWNT/polyvinyl alcohol composite fibers.