In order to test the hypothesis that a combination of blood cortisol measurements and behavioural observations can be used to estimate the avoidance temperature of the estuarine-dependent Cape stumpnose, Rhabdosargus holubi (Steindachner, 1881) (Sparidae), fish were kept at increasing water temperatures at a rate of 3°C d−1 over 7 days. Plasma cortisol concentrations were significantly influenced by the interaction of time and treatment (F = 10.9, P < 0.01) with cortisol concentrations in fish kept at increasing temperature averaging 293 ng ml−1 on day 7. This was 5.4 times higher than the average value for control fish (29 ng ml−1). For the first 6 days, average cortisol concentration of control fish was 25.7 ± 5.0 ng ml−1, while values for fish from the temperature treatment ranged from 8.3 to 176.6 ng ng ml−1. These values combined with behaviour observations suggest that cortisol concentration and behavioural changes may be used to detect both a low and an acute stress response.