The study of pathogenicity of higher plants under conditions of microgravity is of great importance for the future production of food in space. Previous work suggests that microgravity affects both microbes and plants. Bacterial numbers increased after 17 days in an algae-bacterium association on the biosatellite "Kosmos-1887". This was speculated to result from an increase in the multiplication rate of the bacteria. Sporangia of both Actinomices brevis, in the shuttles "Soyuz-19" and "Appolon", and Phycomyces blakes, in biosatellite "Kosmos-936", formed after 10 days in microgravity. Sporangia did not form in the ground controls in the same time suggesting that the rate of fungal development is enhanced in microgravity. Plant responses to pathogens in microgravity have not been studied, however, microgravity profoundly impacts plant cell development, cytology, and physiology. In microgravity, developing cell walls are thinner and contain less lignin than ground-grown plants. The demonstrated effects of microgravity on both plants and microbes lead us to hypothesize that plants may be more susceptible to pathogens under conditions of microgravity. The aim of this study was to determine the influence of microgravity on the susceptibility of soybean to the fungal root rot pathogen, Phytophthora sojae.