The ecology, growth, and behaviour of Metapenaeus mastersii from the Brisbane River, Qld., are described. Postlarval stages were found to favour a warm sheltered locality with abundant algal cover and salinity below 20‰. Salinity tolerance range of all stages was from almost fresh to sea-water, and activity was at a minimum when the average temperature fell to 16�C. Carapace length was found to be as reliable a measure of size as overall length or volume. Progression of the modes of length frequency distributions was unsatisfactory for finding growth rate, but showed a steady downstream movement as growth progressed. From the growth rate of captive prawns, age at sexual maturity was estimated as 12-15 months at 16 mm carapace length (males) and 20 mm (females) (76 and 95 mm overall). Males were less tolerant of low salinity than females, but did not migrate as far downstream. Fertilization occurred in the lower reaches of the Brisbane River, but spawning occurred outside. The length-volume relationship of prawns above 10 mm carapace length (48 mm overall) did not conform to the curve: volume = k × (length)³. This deviation was interpreted as due to change of growth characteristics during winter. A reversed respiratory current and other adaptations to burying in the mud, and a possible associated diurnal rhythm are described.