Project Prairie Grass was a field program designed to provide experimental data on the diffusion of a tracer gas over a range of 800 meters. In each of 70 experiments the gas was released continuously for 10 minutes at a source located near ground level. The gas releases were made over a flat prairie in Nebraska under a variety of meteorological conditions during July and August of 1956. A brief history of the project is included with detailed descriptions of the tracer technique and the meteorological equipment employed in the field program. Tabulations of the diffusion data and the meteorological data collected during the gas releases are also presented. In addition, data is presented on the heat budget at the air-earth interface during other selected periods during the summer of 1956.