The 20-foot aperture horn-reflector antenna at the Crawford Hill Laboratory has been used to measure the flux density of Cas A at 4080 Mc/s. Fifty drift-curve observations were made on six nights in September and October, 1964. In each observation the antenna response due to Cas A was compared with the output of a reference-noise source. The equivalent effective temperature of the reference-noise source was measured by four calibration methods. The gain of the antenna was measured by comparison with a standard horn that was later measured in the laboratory. The resulting flux density of Cas A was 1 086 X 10-23 W m-2(c/s)-1. The probable error in this result is approximately 2 per cent, based upon a 6.3 per cent limit for 99 per cent level of confidence.