Moderate B deficiency in plants has been reported to enhance Cu deficiency by keeping these plants in the vegetative growth stage. In this study, ascorbate oxidase activity was used as an index of the effect of B stress on Cu activity. When T3238FER (B‐inefficient) and Rutgers (B‐efficient) tomatoes(Lycopersicon esculentumMill.) were grown in nutrient solutions at various B levels, B‐stressed plants had higher ascorbic acid oxidase activity than B‐sufficient plants. This activity was significantly higher in T3238FER than in Rutgers. Ascorbic acid oxidase activity may be directly or indirectly related to the role of B in plant growth and may be responsible for the more efficient use of B by Rutgers than by T3238FER tomato.