The rate and duration of the phases of secondary xylem fibre development were quantified at 10 and 60% of tree height from the base in three trees of Eucalyptus globulus. At 60% of tree height the number of cells undergoing division, en1argement and secondary wall development was fewer than at 10% of tree height (P < 0.0001). A very strong positive correlation (r = 0.968, df = 4, P< 0.01) also existed between the rate of radial cell production and the number of cells in the cambial zone. At 60% of tree height cambial derivatives differentiating into fibre tracheids required an average of 5.3 days to complete enlargement and 25.2 days to complete secondary wall development. At 10% of tree height the average duration of these phases was 6.0 and 32.8 days respectively. Although not statistically significant these differences suggest that the duration of fibre differentiation increases basipetally. The average rate of fibre elongation was 66 µm d-1 at 60% of tree height and 99 µm d-1 at 10% of tree height. The duration of the differentiation of vessel elements and cells associated with vessels was shorter than for fibres, indicating that the different cell types in the developing secondary xylem have their own characteristic rate and duration for the processes of differentiation. Considering theories that suggest vessel development is promoted by auxin, this observation is regarded as indirect evidence that auxin reduces the duration of the differentiation process.