Infrasonic signals from thunder have been observed at Palisades, New York, over a number of years. The signals are, in general, dominated by sharp rarefaction pulses with periods in the range of 0.4--1.0 s and peak-to-peak amplitude up to 10 dynes/cm2 (1 N/m2). The signals are found to be highly directional traveling almost vertically downward. Some aspects of these observations are in agreement with the theory of generation of sound by the sudden reduction of electrostatic field within a thundercloud after a lightning discharge, originally proposed by Wilson (1910) and later refined by Dessler (1973). The spectra of the signals also indicate frequencies as low as 0.1 Hz. These low frequencies may be associated with other electrostatic stresses in the thundercloud. A low-amplitude compressional wave at the beginning of the signal is also observed. Sample computations of the thickness of the charged region and electric field strength are given.