A flowing solution culture technique was used to study the effects of N nutrition on banana root infestation by the burrowing nematode Radopholus similis. Paired groups of banana plants were grown in separate compartments containing coarse sand (2-5 mm diameter). Input nutrient solutions had fixed compositions and identical total N concentrations. The two compartments differed in their NH4+/total N concentrations, one receiving 0% NH4+/total N (100% NO3-/total N) and the other 60% NH4+/total N (40% NO3-/total N). The compartments were connected by a inverted T-shaped tube containing fine sand (200-250 µm diameter) through which nematodes were inoculated. The pH and N concentrations of the output solution were determined at regular intervals. The pH of the output solution differed markedly between the two compartments, with values of 6·8 and 3·6 in the 0% and 60% compartments, respectively. In the compartment receiving 0% NH4+, the number of nematodes in the roots and the NH4+ growth of banana plants was significantly higher than in the compartment receiving 60% NH4+. Even though they were more infested by nematodes, banana plants receiving 100% NO3- developed better than the less infested plants grown in the medium containing 40% NO3-.