During the wet summer season of northern tropical Australia higher yields were attained by introduced weakly photoperiod-sensitive short-duration rice varieties than by locally-bred strongly photoperiod-sensitive varieties resembling traditional tropical indicas. There was no clear relationship between growth durations and yields. Dry season yields were lower, associated with shortened durations (especially of the reproductive phase) and lower cumulative radiation totals in the late phases of growth. Dawn, a variety of U.S. origin imported from the Philippines, outyielded all other varieties. In both wet and dry seasons yields depended largely on panicle weights, which in turn depended on high seed numbers per panicle. Individual seed weights were of less importance. The results tended to confirm a prior interpretation of climatic data, i.e. that there is no environmental factor limiting wet season yields in the region to levels below those obtained elsewhere in the tropics, provided the crop receives a high standard of care and attention.