A hybrid swarm between <I>A. brachybotrya</I> and <I>A. calamifolia</I> is analysed by Leach & Whiffin (1978). These hybrids could be referred to <I>A. x grayana</I>. The population of plants studied was at Kiata, Victoria of which <I>A. x grayana</I> was clearly intermediate between <I>A. brachybotrya</I> and <I>A. calamifolia</I> on basis of morphology. Hundreds of hybrids occurred amongst several thousand parental plants. On the basis of phyllode morphology the two parents, the hybrid and two possible backcross groups could be recognised. In addition the flavonoid chemical composition of individual plants was studied and the results may be taken as strong evidence for hybridisation though they are too complex to be elaborated here. The pollen in <I>Acacia</I> is in polyads each containing a regular number of pollen grains. The polyads of <I>A. brachybotrya</I> have 8 and those of <I>A. calamifolia</I> have 16 grains without exception. The intermediates ranged between these (many with 12) but even on one plant 8, 12, 16 grains could be found. Not only did the number of grains support the hypothesis of hybridity but the structure of each polyad and pollen fertility was also consistent. Ecologically the scene is complex with some soil preferences for each species apparent, but overlap did occur and there was also overlap of the flowering times but only of a week or so. The hybrids were intermediate in flowering time and tended to favour <I>A. calamifolia</I> (the earlier of the two). The combined results convincingly support the hybrid origin of <I>A. x grayana</I>.<P><I>A. brachybotrya</I> is a food plant for the larvae of the butterfly <I>Nacaduba biocellata</I>, Fisher (1978). <P><I>A. brachybotrya, A. argyrophylla</I> and <I>A. spilleriana</I> are all closely related and are not always readily distinguished.