The effectiveness of highway noise barriers and the effect of highway noise on high buildings depend on the directivity of the noise source. An experiment was designed to determine the directivity pattern of trucks at highway speeds in a vertical plane. An array of microphones was placed on a vertical tower and was used to record the noise generated by trucks driving by on a nearby freeway. The speed was measured and the characteristics of each vehicle were noted. The results show that trucks exhibit an almost omnidirectional pattern, at least up to 45° from the horizontal, independently of the type of truck or type of exhaust. This conclusion is compatible with the results obtained elsewhere for passenger cars. [This study was supported by the National Cooperative Highway Research Program Project 3‐7. The opinions and findings expressed or implied in this paper are those of the author. They are not necessarily those of the Highway Research Board, the National Academy of Sciences, the Federal Highway Administration, the American Association of State Highway Officials, nor of the individual states participating in the National Cooperative Highway Research Program.]