Tests were carried out to determine the potential hazard of spillage of the pesticides, methyl parathion and ethyl parathion, on concrete surfaces. Results indicated that although a toxic hazard exists, especially for potential contamination of foodstuff, when liquid concentrates of the pesticides are spilled outdoors on concrete surfaces, this hazard can be considered to have disappeared within 3 months for methyl parathion, but may persist for over 15 months for ethyl parathion. The hazard with more persistent organophosphorus pesticides, such as ethyl parathion, may be increased after outdoor exposure of concrete surfaces to cold damp weather. If the spill occurs inside, the toxic hazard for methyl parathion persists approximately twice as long as for an outside spill. It is assumed that the inside hazard for ethyl parathion would also be extended for a considerable time compared to the outside hazard. For more persistent pesticides, in lieu of the almost impossible task of removing the concrete surface to a considerable depth, a physical sealing of the surface may prove effective if suitable chemical decontamination cannot be accomplished. Additional research is being carried out to explore this possibility.