The brilliant blue fruit color of Delarbrea michieana (F. Muell.) F. Muell. (Araliaceae), a Queensland understory rainforest tree, is caused by a structure (an iridisome) in the epidermal cells. The iridisome is produced beneath the cell wall and above the cytoplasm, although its relationship to the plasmalemma is uncertain. Layers within the iridisomes are of the thickness to interfere constructively with light at 420-440 nm and produce the blue color. This structural coloration is unique to this single and uniquely Australian species, among six taxa in the genus. The iridisome may be of relatively recent origin. Iridescent blue color production in this fruit may aid in dispersal by mammals and frugivorous birds and may allow ripe fruits to continue photosynthetic carbon assimilation.

Key words: constructive interference, fruit, tropical rainforest, ultrastructure