Carbon nanotube-polymer composite fibers are obtained by infiltration of a monomer liquid into aligned carbon nanotube aerogel fibers with subsequent in situ polymerization. The monomer, methyl methacrylate (MMA), was infiltrated into the aerogel fibers of multi-walled carbon nanotubes (MWNTs) at room temperature and subsequently polymerized at 50 degrees C into poly(methyl methacrylate) (PMMA). Cross-sections of the PMMA/MWNT composite fibers showed that the PMMA filled the spaces of the nanotube fibers and bound the nanotubes together. PMMA in the composite fibers exhibited local order. The resultant composite fibers with 15 wt.% nanotube loading exhibited a 16-fold and a 49-fold increase in tensile strength and Young's modulus, respectively, compared to the control PMMA.