In spite of its wide distribution in arid Australia and its importance to the pastoral industry, mulga (Acacia aneura Benth.) has not been the subject of any detailed investigations to determine the climatic requirements for regeneration. An investigation of these requirements for flowering and seed set, in which trees growing in the field were given additional water for a period of 12 months, showed that flowering occurred several times in the year but that the principal flowerings were in spring and late summer. It was not shown definitely that rain was necessary to bring about flowering at these seasons, although the results suggested that flowering was heavier and seeding more successful when additional water, or good rains, reduced the normally severe water stress. Of the two principal flowering seasons, only the late summer one led on to seed set and the shedding of ripe seed. From rainfall records and values for the climatic requirements derived from observations made during the investigation, it was estimated that seed could have been set successfully once every six years in the experimental area since records began around 1890.