WHEN a material absorbs a photon, a fraction of the energy may be transformed into heat. A measurement of photothermal heating as a function of wavelength can provide an absorption spectrum of the material. We have recently(1,2) developed a micromechanical sensor capable of detecting heat changes of the order of picojoules (10(-12) J). The instrument incorporates a bilayer cantilever of micrometre dimensions which bends in response to heating. Here we show that this device can be used for photothermal spectroscopy with a power sensitivity of 100 pW-two orders of magnitude better than the sensitivity of conventional photothermal deflection spectroscopy(3). The small size of the sensor allows picogram quantities of material to be studied, opening up the possibility of spectroscopic studies on individual cells and bacteria. Being based on silicon technology, the sensor should be compatible with microelectronic circuitry.