The effect of a pilot flame on the dynamic stability characteristics of a single-nozzle lean-premixed swirl combustor operating on natural gas fuel is investigated. Experiments are performed at a fixed inlet temperature and inlet velocity over a broad range of overall fuel-lean equivalence ratios with varied amounts of pilot fuel. The stability results are presented in the form of a dynamic stability map which is a three-dimensional graph of the normalized rms pressure fluctuation versus the overall equivalence ratio and percent pilot. The dynamic stability map identifies boundaries between stable and unstable combustion and results are presented characterizing the transition across one of these boundaries. Time-averaged two-dimensional chemiluminescence images of the stable flame and phase-averaged two-dimensional chemiluminescence images of the unstable flame are used to gain an understanding of the effect of the pilot on dynamic stability through changes in flame structure.