The compression micromacrostrain region has been examined in continuous fiber composites of magnesium-70 volume per cent boron and copper-23 volume per cent tungsten at room temperature. Composites were examined in the as-received, annealed, and prestrained about 3 × 10⁻³ conditions. In the as-received condition, both composites exhibit initial elastic moduli similar to or greater than the theoretically predicted values; the initial moduli are sensitive to annealing and prestrain or both. The precision elastic limit is increased as a result of fiber reinforcement but then shows a decrease as a result of prestrain and annealing; magnesium-boron is particularly sensitive to annealing. The complex stress-strain relationships in the microstrain region are discussed in terms of the interface bond, the possible role of microcracks and porosity, spiralling of the fibers, and the compressive mode of loading. The microstrain technique is established as an effective approach to an evaluation of the micromechanical properties of composite materials.