The enhancement of turbulent flame development using a microwave-assisted spark plug has been investigated in methane-air and methane-oxygen-argon mixtures, where the mole fraction of argon in the oxygen-argon mixture is the same as that of nitrogen in air. Experiments are conducted using a 1.45 liter constant volume combustion chamber with a fan mounted inside to promote turbulence. Experiments were performed for various levels of microwave energy input at a range of equivalence ratios and turbulence intensities. Microwave enhancement was evaluated on the basis of flame development time (FDT), flame rise time (FRT), total net heat release, and ignitability limit extension. The addition of microwave energy to a capacitive discharge spark decreased the FDT at lean and rich equivalence ratios for turbulent methane-air mixtures at atmospheric pressure, with the greatest effect observed at slowerburning conditions. A similar trend was observed for turbulent methane-oxygen-argon mixtures. Negligible enhancement was observed at the fastest burning conditions for both mixture types. Increasing microwave energy input further reduced the FDT for the lean/rich methane-air mixtures tested, but different thresholds were observed above which additional microwave energy did not further reduce the FDT. Increasing turbulence intensity diminished flame enhancement from microwave energy deposition, however, for a given FDT, turbulent conditions resulted in greater flame enhancement compared to laminar conditions. Additionally, the rich and lean ignitability limits were extended using the microwaveassisted spark plug compared to a "spark only" ignition. The addition of microwave energy had no effect on flame rise time or on total net heat release for the conditions studied.