Target strengths of a submerged target, believed to be a sperm whale,Physeter catodon, were measured with the use of a calibrated passive sonobuoy and explosive charges. Measurements of 148 sperm whale echolocation clicks in a13‐octband having a center frequency of 1 kHz resulted in an average source level of 73.5 dB // 1 dyn/cm2with a standard deviation of 3.51 dB.