Piezoresistivity in unidirectional carbon fiber epoxy-matrix composites was studied by measurement of the electrical resistance of single-lamina and two-lamina composites in the longitudinal (fiber) direction during elastic tensile loading and unloading in the same direction. The piezoresistive effect was reversible. The resistivity increased slightly with longitudinal tensile strain for the single-lamina composite, but decreased with strain for the two-lamina composite. The gage factor was around +2 and -6 for single-lamina and two-lamina composites, respectively. The very weak piezoresistivity in the single-lamina composite was probably due to decrease in the extent of fiber-fiber contact upon tension. The relatively strong piezoresistivity in the two-lamina composite was probably due to increase in the degree of fiber alignment upon tension.