Levels of phenoloxidase (Po), trypsin and lysozyme activities were determined following 48 hours exposing freshwater crayfish (Pontastacus leptodactylus) weighing 59.8±3.4g to different low pH values of 6.5, 5.5, 4 and 2.5 at 20-23˚C. Significant decrease were found (p<0.01) in levels of Po and trypsin activities in crayfish haemocyte lysate supernatant (HLS) at pHs 5.5 and 4 compared to the control group (pH 8). Also, plasma lysozyme activity showed a significant increase in water with pHs of 6.5, 5.5 and 4 (p<0.01). Furthermore, measurement of calcium and magnesium concentrations in sera samples showed a significant decrease for calcium ion at pHs 5.5 and 4, while magnesium concentration was significantly decreased only at pH 4 compared to control group (p<0.05). Crayfish exposed to pH 2.5 died within 24 hours post-exposure.