The design and properties of thermal detectors constructed with multiple layers of polyvinylidene fluoride film are described. By use of these detectors, production of heat by the photoreceptors in the darkadapted bullfrog retina in response to brief pulses of very weak light was examined. In response to brief light pulses delivering an average of 1 photon per retinal rod, the retina was found to produce heat more-or-less abruptly after a latent period of 0.2 to 0.4s. At this level of pulse intensity, the thermal energy produced by the retina was approximately 1, 500, 000 times as large as the total radiant energy delivered to the retina for stimulation. It appears possible that a large number of disks in the rod outer segment are activated by absorption of a single photon.