A hybrid swarm betweenAcacia brachybotryaandA. calamifolia,involving plants referable toA. grayana,was studied in detail. Plants representing the range of variation found within the hybrid swarm were examined morphologically, chemically and ecologically in an attempt to document hybridisation and to determine their status. Plants referable toA. grayanaare confirmed as hybrids, and other intermediate plants are determined as backcrosses to either parent. Phyllode morphology, pollen polyad form and flavonoid composition prove extremely useful in identifying the various elements of the hybrid swarm. Various other aspects of the biology and ecology of the plants in the hybrid swarm are noted.