Features of the autecology of four native perennial grasses (Stipa, Chloris, and Digitaria spp.) and an important group of winter annual species are reported from a 5-yr study of a naturaI pasture at Trangie, N.S.W. The investigation was mainly concerned with the response of the species to changes in seasonal climatic conditions. The study period (1948–52) was relatively moist (mean annual rainfall 25.6 in.) and the growing season (May–November) longer than average. Available moisture showed considerable variation in autumn and spring, and was the primary factor in the variable success of germination and seedling survival among the perennial grasses. Plant development in the perennial grasses is discussed for three study intervals, March 1948–50 (above average rainfall), March 1950–51 (recurrent flooding over 8 months), and March 1951–52 (including a long, dry summer) ; in all pkriods striking fluctuations were noted in persistency and basal area of established plants, particularly the small plants of each species. Available moisture is advanced as the principal factor controlling seasonal distribution of green forage production by the perennial grasses, temperature becoming limiting only at midwinter (June–July). All species conformed to a similar production pattern, showing a midsummer minimum (except in January 1951), late summer and spring peaks, and a midwinter trough. Flowering and maturation periods are listed for the perennial grasses, and infrequent but heavy seed production recorded for all species. Characteristic features of the root systems are discussed in relation to drought resistance and response to effective rainfall. Germination of Medicago spp. was most frequent in May and June (1-in. soil temperature 51–61°F), but was also successful in March, April, and July. Erodium cygnorum and Hordezcm leporinum favoured late February to early May (65–75°F) for germination. Among high populations percentage seedling survival was low, even in a very favourable season. Given adequate winter moisture (as in 1948 and 1949), Medicago spp. and E. cygnorum showed virtually an exponential increase in green forage production between May and September. The level of September production of these species is shown to be correlated with moisture availability in April. The principal factors ensuring regular recurrence of the winter annuals in pastures of the Trangie environment are outlined and discussed.