As part of a summer undergraduate research program (STEP) at Truman State University, and funded by the National Science Foundation, the crystal and three-dimensional structures of two organophosphorus compounds were determined by X-ray diffraction. The organophosphorus pesticide of primary interest is 2-(diethoxy-thiophosphoryloxy)-7-methyl-pyrazole[1,5-a]pyrimidine-6-carboxylic acid ethyl ester (also known as Pyrazophos and AfuganŽ) C₁₄H₂₀N₃O₅PS, a systemic fungicide. Pyrazophos exhibits 2 rotamers per asymmetric unit, and a carbonyl---ring p system interaction between the carbonyl oxygen and the ring center. This interaction occurs along the b-axis and is present in only one of the rotamers. Also of note is a discrepancy between the observed placement of a ring methyl position and the placement as seen in the current literature. O-2,6-Dichloro-p-Tolyl O,O-di-methylphosphorothioate (also known as Tolclofos-Methyl) C₉H₁₁Cl₂O₃PS, was also determined during the program, and of interest in this compound is libration about the central phosphorus atom resulting in partial occupancies of the two methoxy groups.