Eucalyptus calophylla and other species of series Corymbosae are shown to produce functionally male flowers by abortion of the structures of the ovary during development. In their extreme form male flowers may be recognized by their shape but others outwardly resemble bisexual flowers. In E. calophylla some male flowers give rise to fruits which are aberrant in shape and seedless. The ratio of male to bisexual flowers varies from tree to tree. Some trees appear to produce only male flowers. The morphology and anatomy of male and bisexual flowers are described and compared. The oil duct systems of male flowers are not fully developed. The evident trend towards polygamy in some species of series Corymbosae (and in E. calophylla towards androdioecism) raises the question of its adaptive and evolutionary significance.