Fungitoxicity, in terms of inhibition of mycelial growth, was demonstrated with eight chlorinated hydrocarbon insecticides by the culture plate bio-assay method. All of the dosage–response curves thus derived were bimodal rather than linear. Evidence is presented to show that the shape of these curves is governed by two physical properties of the compounds: water solubility (which was determined experimentally) and vapor pressure. The concentration of insecticide in solution governs the initial rise of each dosage–response curve which is extended to a peak because of supersaturation and is followed by a decline as normal saturation is regained. The concentration of toxicant vapor in contact with the organism governs the second rise in the curve to a level dependent on the vapor pressure of the compound. Lindane has a high degree of fungitoxicity because of its relatively high water solubility. Heptachlor, chlordane, and aldrin are also highly toxic because of their vapor pressures. DDT, methoxychlor, dieldrin, and endrin are weakly fungitoxic because of both low water solubility and low vapor pressure.