The influence of Co on Fe uptake and the Fe‐stress response mechanism of tomatoes (Lycopersicon esculentumMill.) and soybeans [Glycine max(L.) Merr.] was studied. Iron‐efficient and ‐inefficient plants (T3238fer+and T3238fertomatoes and A7 and T203 soybeans, respectively) were grown in nutrient solutions for seven and nine days at varying Fe and Co levels (0, 0; 1.0, 0; 1.0, 0.3; 1.0, 0.6; 1.0, 1.2; 1.0, 2.4; and 0, 1.2 mg liter–1Fe and Co, respectively). Cobalt significantly (p≤0.05) decreased chlorophyll and foliar Fe and increased foliar and root Co in all cultivars. Cobalt effects on root Fe varied among cultivars. The Fe‐inefficient cultivars were most susceptible to chlorosis in the presence of Co, yet apparently exhibited true Fe deficiency. The Fe‐efficient cultivars appeared to exhibit a combination of Fe deficiency and Co toxicity. Chlorophyll formation was generally limited when foliar Fe was below 50 ug Fe g–1dry weight regardless of Co treatment. Reductant and proton excretion from roots of efficient cultivars apparently did not occur at Co levels where Fe uptake was limiting chlorophyll formation. However, the Fe‐efficient cultivars appeared to remain efficient at low Co levels even though the outward measurable indicators (pH and excreted reductant) were not evident. The Fe‐stress response mechanism was not observed in Fe‐inefficient cultivars regardless of Co level.