The Tamar Estuary, Tasmania, has a tidal regime transitional between semi-diurnal and diurnal with the characteristic tidal sequence being low low, low high, high low, high high. In the mature Rosevears marsh, Spartina covers the high tide flat sloping at 0.5¬∞ and extends down the upper third of the intertidal slope at 5.0¬∞, to a sharply defined continuous lower limit with only isolated clumps to 0.1 m below. During 1977 the whole marsh was immersed on all high tides, to a depth between 0.2 and 1.0 m at its upper limit. The lower limit lay at the height of the highest, high low tides but fully emerged on all low, low tides. The upper, middle and lower parts of the marsh were immersed for 1830, 3970 and 6650 hours respectively in 1977. Maximum periods of 23 hours continuous immersion of the lower edge occurred mainly in winter, resulting in non-emergence during daylight hours for up to five consecutive days. The sharp continuous lower limit may result from insufficient light under these conditions to enable buds to form to produce shoots in the following summer. Also, increasing instability in the substrate down the intertidal slope will prevent seedlings and shoots becoming firmly rooted.