Abstract Fishes are often stocked outside natural distribution ranges with inadequate information on target streams, particularly thermal regimes. Australian bass, Macquaria novemaculeata (Steindachner), is a catadromous species that is regularly stocked into upland reaches of rivers and impoundments in south-eastern Australia. Critical Thermal Minima (CTMin) were determined for age-0 Australian bass fingerlings with a mean fork length of 64.4 ± 0.4 mm and weighing 3.8 ± 0.8 g. Four treatments were used, including three replicate aquaria for each treatment. Fingerlings were acclimated at either 8 or 15 °C at densities of 15 fish in 56-L glass aquaria. Water temperatures were then decreased at either 1 °C day−1 or 1 °C h−1 until loss of equilibrium (LOE), which occurred between 3 and 7 °C. Mean CTMin among treatments was 3.22–4.64 °C and was influenced by acclimation temperature and rate of temperature decline. Fingerlings acclimated at 8 °C subjected to a temperature decline of 1 °C h−1 experienced highest LOE temperature. Post-LOE mortality among treatments was highest at 100% in the 8 °C acclimation with a 1 °C day−1 temperature decline. Mortalities following LOE occurred within 5 days. The results suggest that stocking age-0 Australian bass is unlikely to be successful in areas where winter temperatures fall below 6 °C.