Vapor-grown carbon fibers (VGCFs), a practical model nanofiber for single-walled carbon nanotubes, were combined with an acrylonitrile-butadiene-styrene (ABS) copolymer to create a composite material for use with fused deposition modeling (FDM). Continuous filament feedstock materials were extruded from Banbury mixed composites with a maximum composition of 10 wt % nanofibers. Issues of dispersion, porosity, and fiber alignment were studied. SEM images indicated that the VGCFs were well dispersed and evenly distributed in the matrix and that no porosity existed in the composite material following FDM processing. VGCFs aligned both in the filament feedstock and in the FDM traces suggested that nanofibers, in general, can be aligned through extrusion/shear processing. The feedstock materials were processed into test specimens for mechanical property comparisons with unfilled ABS. The VGCF-filled ABS swelled less than did the plain ABS at similar processing conditions due to the increased stiffness. The tensile strength and modulus of the VGCF-filled ABS increased an average of 39 and 60%, respectively, over the unfilled ABS. Storage modulus measurements from dynamic mechanical analysis indicated that the stiffness increased 68%. The fracture behavior of the composite material indicated that the VGCFs act as restrictions to the chain mobility of the polymer. (C) 2003 Wiley Periodicals, Inc.