The haematological response to ambient (10µg Cd l−1 ) or dietary (10µg Cd fish−1 day−1 ) cadmium for 2 and 14days was investigated in tilapia (Oreochromis mossambicus) acclimatised to low-calcium (0.2mM Ca2+) and high-calcium water (0.8mMCa2+). Significant reduction of erythrocytes (red blood cells), haemoglobin (Hb), packed cell volume (PCV), mean cell haemoglobin (MCH) and mean cell volume (MCV) occurred in fish exposed to ambient cadmium in lowcalcium water. The anaemic response of significantly reduced Hb, PCV, MCH and MCV was also evident in fish exposed to dietary cadmium in low-calcium water. Except for increased mean cell volume of erythrocytes, both ambient and dietary cadmium had no effect on haematological indices in fish from high-calcium water. This study indicates that high-calcium water has a protective effect on cadmium toxicity.