Spores of Alternaria solani carried on glass fiber filter disks were employed in a study of the fungicidal activity of the fumigant, methyl bromide. Following exposure to the fumigant the disks, carrying the spores, were placed in a spore germination medium and the results subsequently evaluated by direct mortality counts. Linear probit-log dose regression lines were obtained by this procedure. The effectiveness of the fumigant was found to be dependent on the vapor pressure of water in the atmosphere and to be independent of temperature. The results suggest that humidity operates through its effect on spore hydration.