Josephson junctions connected to high Q microstriplines display current-steps which we attribute to standing wave resonances. The step voltages of the resonances depend on sample geometry and correspond to frequencies at which the loaded microstrip is resonant. The voltages of the resonances are insensitive to applied magnetic field and junction current density. Current step amplitudes scale with the zero-voltage critical current, and are largest for zero applied magnetic field. The strength of these resonant steps has important implications for biasing weakly damped Josephson junction circuits. In addition, the resonances are a useful tool for measuring microstrip properties.