The first determination of the density, pressure, and particle distribution in a lightning stroke near peak temperature is presented. Relative amounts of atomic nitrogen, atomic oxygen, and singly ionized nitrogen are determined from line-intensity measurements in an optical spectrum of a return stroke. This information in conjunction with the previously measured stroke temperature from the same spectrum and theoretical tables of the thermodynamic properties of air is sufficient to determine the following properties of the lightning stroke: at a temperature of 24,000K the mass density of the stroke is about 0.1 of the mass density of air at STP; the stroke pressure due to atoms, ions, and molecules is about 18 atmospheres; the stroke pressure due to electrons is about 14 atmospheres; and the electron density is about 4.3×1018 cm−3. The number of electrons per air atom (neutral, ionic, or comprising a molecule) is about 0.81; analogous quantities for 25 species of air particles present in the stroke at 24,000K are presented. The validity of the assumptions used in the analysis is considered.