The buds of Eucalyptus regnans are protected by an operculine involucre for 12 months and develop as exposed umbels for a further 15-17 months. The flowering period is about 3 months but the extreme range is from late February to early July at elevations of 600-750 m. Flowers remain open for 1-2 weeks. Older trees tend to flower later than younger trees on the same site. Pollination is probably effected chiefly by insects and birds. Flowering abundance follows a 2-year cycle with a tendency for heavier production every fourth year, but this pattern may be interrupted by severe insect attack on young shoots. Considerable losses occur at all stages of development owing to abscission, and to damage by gall and other insects. Quantitative estimates of flowering from litter trays indicate that the flower production of good years may be 25-65 times as great as in poor years. The mature forest produced 2.15-15.5 times as many flowers as the pole stage trees, and 1.5-10 times as many as the spar stage forest. Estimates of the fruit set following the late autumn flowering of 1954 indicate that that of the mature forest was 1.6 times as great as that in the spar stage forest and 3.5 times as great as that in the pole stage forest. The corresponding relative flower abundance in these forests was 1.5 and 3.8 respectively. However, the percentage of Rowers setting fruit from the early autumn flowering of 1956 was very much lower. Most capsules abscind or dehisce or fall attached to twigs within 3 years of their maturity. The quantitative estimate of flowering and fruit set permits seed crops from heavy flowering to be forecast a few years ahead of seed fall.