Summary The infection of the tropical legume Leucaena leucocephala by the broad-host-range, fast-growing Rhizobium strain NGR234 was studied using both light and electron microscopy. Rhizobium infection of Leucaena was shown to be via infection threads formed in root hairs. Electron microscopic studies revealed both inter- and intra-cellular infection threads in nitrogen-fixing nodules. Root hair deformation and infection was mostly on lateral roots but occasionally found on the taproot. The lateral roots appeared on plants 3 days after two and half day-old seedlings were transferred to the growth medium in the assay vessel. Root hair curling or branching was first observed 5 days after Rhizobium inoculation. Infection threads were seen on each plant between 10-18 days after inoculation. Nodules were visible on plants 12 days after inoculation and most of them were located on lateral roots.