Biochemical and histochemical analysis of soybean [Glycine max (L.) Merr.] cv. Giza 111 plants naturally infected with Fusarium virguliforme were studied to clarify the susceptibility mechanisms under open field conditions during 2015 growing season. Levels of superoxide (O₂^•−) and hydrogen peroxide (H₂O₂) which are the major reactive oxygen species (ROS) forms were significantly increased and accumulated after 3 and 5 days from symptom appearance in infected soybean plants. Also, electrolyte leakage (EL) values which indicate to the membrane permeability were increased after 3 days from symptom appearance, while the activities of catalase (CAT), peroxidase (POX) and polyphenol oxidase (PPO) enzymes were decreased. In addition, concentrations of chlorophyll A and B were significantly decreased in infected soybean plants in comparison with the uninfected one. Down-regulation of antioxidant enzyme activities might be the key role of soybean susceptibility to sudden death syndrome (SDS) infection which caused by soil-borne pathogen F. virguliforme. Antioxidant enzyme activities in infected plants were not enough for ROS-scavenging and initiated the programmed cell death and led for sudden wilt of susceptible plants. Plants showed wide range of defense responses in reaction to initiated invasion by the pathogen, including the production and accumulation of ROS by the process called "oxidative burst" which is the rapid and earliest step to produce large amounts of ROS and use them as weapons in a defense response system. Reactive oxygen species work also as signals which are able to stimulate other plant defense mechanisms. Steps of reactive oxygen species production and scavenging are highly dynamic and consists of a complex signaling network. Giving more attention to antioxidant enzyme activities and reactive oxygen species levels is needed for plant breeders in order to create a resistant soybean cultivars and might be useful for discovering a new alternative disease control strategies in which safety and decrease the environmental pollutions.