Transgenic tomatoes (Lycopersicon esculentum Mill. `Ohio 8245') expressing an antisense catalase gene (ASTOMCAT1) were used to test the hypothesis that modification of the reactive oxygen species scavenging mechanism in plants can lead to changes in oxidative stress tolerance. A 2- to 8-fold reduction in total catalase activity was detected in the leaf extracts of transformants. A 2-fold increase in levels of H2O2 was observed in the transgenic plants with reduced catalase activity. Electrophoretic characterization of multiple catalase isoforms revealed the specific suppression of CAT1 in transgenic plants. Homozygous plants carrying the antisense catalase transgene were used to study the effect of alteration in the expression of catalase on stress tolerance. Transgenic plants treated with 3% H2O2 showed visible damage within 24 hours and subsequently died. In contrast, wild-type and azygous control plants recovered from the treatment. Transgenic plants did not survive 4 °C chilling stress compared to control wild-type and azygous lines. Physiological analysis of these plants indicated that suppression of catalase activity in transgenic tomato led to enhanced sensitivity to oxidative stress. Our data support a role for catalase in oxidative stress defense system in tomato.