The population structure and reproductive biology of the river oak (Casuarina cunninghamiana) were studied along the Murrumbidgee River on the Southern Tablelands of eastern Australia. The species has cone-like infructescences but is not serotinous, with samaras released when they are mature. Samaras were mostly distributed by wind close to female trees but were also carried by water. The first year of study (1985) appeared to be a mast year for seed production with much lower seed fall in the following year. Seedling establishment was spatially very variable, mostly under female canopies and appeared earlier on soil within the river channel than on the bank. Turnover was high and seedlings in the river channel generally died after being inundated. Most trees were within 3 m of the mean river level. However, the total distribution of adults was within the envelope of maximum floods in the area but establishment was not dependent on floods. The population structure was the result of yearly recruitment, although episodic events (floods, drought) may enhance or decrease establishment. Pot-trial results paralleled the field situation with substrate and water-table level not affecting germination of seed but strongly influencing seedling growth. Plants grew best on cobble substrates under a low water-table regime and poorly on cobbles with high water and sandy substrate under all water-table levels. Cobble banks seemed the best substrate for growth within the river channel and establishment may be prolific. Less-abundant seedling establishment occurred upslope but controls over this were not investigated.