Piezoresistivity was observed in continuous unidirectional carbon fiber cement-matrix and polymer-matrix composites. The fiber volume fraction was 2.6-7.4% and 58% for cement-matrix and polymer-matrix composites respectively. The DC electrical resistance in the fiber direction increased upon tension in the fiber direction for the cement-matrix composite, due to fiber-matrix interface degradation, but decreased upon tension for the polymer-matrix composite due to increase in the degree of fiber alignment.