Micronutrients, which are often deficient in volcanic soils, together with macronutrients may lead to higher yields in these soils. A study was conducted under pot and field conditions to identify and correct some micronutrient constraints in a volcanic soil at Mpangala, Tanza- nia, for optimization of maize (Zea mays L.) yields. Dry matter (DM) yields, plant B, Cu, and Zn concentrations, plant B, Cu, and Zn uptake, and grain yields were used to assess the effects of micronutrient treatments. In pots, B, Cu, and Zn fertilizers were applied separately to the soil at two levels, 0 and 2 mg B kg 21 , 0 and 5 mg Cu kg 21 , and 0 and 10 mg Zn kg 21 , in combination with constant rates of 240 mg N kg 21 or 160 mg P kg 21 fertilizers. A higher rate of 320 mg P kg 21 was also included to assess the adequacy of the basal P rate used. A second pot study attempted to establish an optimum rate of Cu under glass- house conditions; rates ranging from 0 to 20 mg kg 21 Cu were tested. Copper significantly (P 5 0.05) increased both maize DM and grain yields; the estimated optimum rate was 20 mg Cu kg 21 under glass- house conditions. This high rate is thought to be due to the high Cu- fixation capacity of volcanic soils. Boron and Zn were sufficient for normal plant growth. We conclude that maize production can be in- creased considerably in Mpangala and other similar soils in the same agroecological zone by applying N, P, and Cu at rates of 120, 80, and 10 kg ha 21 , respectively.