Abstract  The effects of the nutrient solution oxygenation on the growth of tomato plants and colonization of plant roots by Pythium F707, an isolate with filamentous non-inflated sporangia, were investigated under hydroponic conditions. Lipoperoxidation was also estimated determining lipoxygenase activity and conjugated dienes. Tomato plants were grown under either a high (11-14%; Air treatment), a moderate (5.8-7%; Control) or a low (0.8-1.5%; Nitrogen treatment) oxygen concentration and inoculated or not with the pathogen. The high oxygen treatment resulted in a marked increase in plant growth, as measured by shoot and root weights. Root and top weights were about the same in the nitrogen-treated plants and the controls. In these treatments, plants started showing typical symptoms of root decay and infection within 6 days after inoculation with Pythium F, while highly oxygenated plants remained healthy throughout the experiment and showed a significant decrease in root colonization by the pathogen, as estimated by the immunoenzymatic staining procedure and isolation of thalles on selective medium. Nitrogen-treated plants and controls produced higher amounts of conjugated dienes and revealed increased lipoxygenase activities in comparison with highly oxygenated plants. These differences were more pronounced after inoculation with the pathogen. Our data suggest that increases in lipoxygenase activity detected in the present study in tomato roots grown under oxygen stress and inoculated with Pythium F may lead to degradation and disorganization of membrane lipids. That disorganization may facilitate root colonization by the pathogen and appearance of decay.