Two naturally occurring micas, phlogopite and muscovite, were ground and classified according to their average aspect ratios (flake equivalent diameter to thickness ratio). These flakes were then used to reinforce a polystyrene copolymer and a polyester resin. The compression molded test pieces were tested in flexure and the flexural strengths and flexural moduli determined for each aspect ratio. The experimental results indicate a strong dependence of strength and modulus on the flake aspect ratio up to a value of 100–200 for these systems. At high volume fractions, 0.6 to 0.7, high aspect ratio mica composites yielded flexural strengths of 35–45,000 psi with flexural moduli of 10–14 million psi. Notched Izod values were in the range of 0.5–1.4 ft lb per inch of notch. These results were compared with the theoretical treatments of Padawer and Beecher, and Riley.