To date, integrated waveguides with the lowest losses have all relied at least in part on amorphous materials. In this work, we demonstrate fully crystalline, single-mode integrated microresonators comprising epitaxially grown Si0.86Ge0.14 waveguide cores with silicon claddings. This platform supports ultrahigh-quality-factor resonances, with Q reaching 1.71 ± 0.06 × 108, corresponding to a loss rate of 0.39 ± 0.02 dB/m. This Q is nearly an order-of-magnitude improvement over that seen in prior integrated Si waveguides. Together with silicon's strong Kerr nonlinearity, our results could unlock important new avenues for microwave photonics, optomechanics, and quantum transduction.