An ac susceptometer has been developed for very sensitive measurement of the magnetic moment of small samples as a function of temperature and applied dc field. The system was designed specifically for measurements on small single crystals of the high Tc superconductor YBCO in the Meissner state. The natural platelet geometry of these crystals allows results to be easily transcribed into changes in the material's magnetic field penetration depth, an important quantity in the exploration of the underlying physics of its superconductivity. The resolution of the technique is of the order of one tenth of an angstrom for a sample of area ~2 mm².