Electron spin resonance (ESR) of phosphorus-doped silicon was detected using a low temperature magnetic resonance force microscope (MRFM). Force-detected ESR spectra were obtained using an amplitude or frequency modulated microwave field to cyclically saturate the spin magnetization. For a sample containing4×1018phosphorus atoms/cm3, a single strong ESR line was observed. For a sample containing8×1016phosphorus atoms/cm3, a pair of lines split by the 42 G31Phyperfine interaction was observed. This result demonstrates the possibility of using MRFM techniques for spectroscopic purposes. ©1997 American Institute of Physics.