The development of young mycorrhizal root systems (up to 28 d) has been studied by growing Trifolium subterraneum L. in soil/sand mixtures with mycorrhizal inoculum mixed throughout the pots. Rates of initiation and of apical extension of main and first- and second-order lateral roots were calculated by fitting spline functions to data collected from daily harvests. The average instantaneous rates of apical extension were found to be about 0.77, 0.2 and 0.06 cm d−1 respectively. These values were used, together with the size and distance from the root apex of the most apical mycorrhizal infection unit, to calculate the frequency of mycorrhizal infection close to the root apex (At) at 16 d. Values of At were found to be similar to average values for the whole main or lateral root system (A). We used plots of the size of infection units and their distances from the root apex, together with values of average rate of extension of roots (v) and rate of growth of infection units (B), to calculate the distances between entry points and the root apices at the time the entry points were formed. The results indicate that infection can occur not only immediately behind the root apex, but also to considerable distances (up to 55 mm) behind the apex in 16-d-old plants.