The thermal conductivities of plastically deformed KCl crystals were measured in the temperature range 0.0210 K. The measurements indicate that thermal phonons are scattered strongly by dislocations. Heat-pulse measurements support this interpretation. The thermal conductivity of a deformed KCl sample is restored by irradiation, suggesting that the phonon-dislocation interaction in KCl is dynamic in nature. The experimental results are compared with numerical calculations assuming dynamic phonon scattering from isolated, fluttering dislocations. There is reasonable agreement for small dislocation densities, but not for heavily deformed samples.