Tm speed of sound in a solid is altered in the presence of mechanical stress. This effect can be exploited to form the basis of ’acoustic microscopy’, whereby images of stress patterns in a material are obtained hy monitoring the times of flight of sound pulses’. The sensitivity of this approach is limited because wave speeds typically change by less than 1%, even when materials are stressed until they yield. Here we demonstrate a more sensitive form of acoustic microscopy. In an isotropic elastic medium, stress-induced anisotropies affect wave polarizations and phases, giving rise to interference between waves that would, in the absence of stress, remain in phase; the resulting patterns of interference between these waves reveal the underlying patterns of stress. This technique, by using acoustic waves of different wavelengths, permits the imaging of stress in objects ranging in size from microelectric devices to welds in pressure vessels.