The response of carob (Ceratonia siliqua L.) seedlings grown at different root zone temperatures affected by nitrate and ammonium nutrition was studied. When root temperatures ranged from 10 to 35°C, ammonium-fed plants were significantly larger than nitrate-fed plants. Ammonium-fed plants displayed toxicity symptoms and were much smaller at 40°C root temperature in comparison with the nitrate-fed plants grown at the same root temperature. Root/shoot ratio slightly increase with root temperature in ammonium- and nitrate-fed plants in a similar way, and shoot demand per root unit decreased with root temperature between 15 and 25°C. There was a general increase in net photosynthesis with root temperature, though nitrate-fed plants were more sensitive to low and ammonium-fed plants to high temperatures. Increasing the root temperature of ammonium fed plants from 10 to 40° C leads to a 30% increase in the amount of photosynthates sent to the roots. The presence of ammonium resulted in the distribution of newly fixed carbon away from carbohydrates and into nitrogen compounds. Potassium, calcium, and nitrogen content of the plants also increased with increasing root temperature.