In the tropical region of north-eastern Australia flowering occurs in late summer and short days, since the wet season prevails over the second half of the summer. When northern material is grown in uniform grass gardens south of this tropical region, late maturity still prevails; hence this behavioural pattern is genetically fixed. In the subtropics, on the other hand, adequate rain may fall over the entire potential growing season or any part of it. In the garden great variation of phenology marks the southern material. Furthermore, sampling within local populations may also show great variation from early maturity through to late. It is suggested that in Australia H. contortus is naturally a species of the semi-arid tropics. It has spread into the subtropics through its ability to match the extended but more uncertain rainfall season by producing types of differing day length response. This variation is fixed by the predominantly apomictic mode of reproduction, while the perenniality of the plants ensures survival over periods of unsuitable climate. A long-term equilibrium is therefore established between the flowering behaviour of the population and its climate.A knowledge of the patterns of behaviour within and between populations is likely to be Important for increased pastoral efficiency, firstly, in preventing natural populations from deteriorating towards earlier maturity, and secondly, in lengthening the productive season of pasture growth.