Early maturity in rainbow trout (Oncorhynchus mykiss) is an economic loss for fish production industry since flesh quality and appearance get deformed as metabolic energy is directed from somatic growth into reproduction. Triploid fish production dramatically reduces this situation, even if it does not fully wipe it out. However, the resistance of triploid fish against different stress factors resulted from aquaculture conditions is contentious compared to diploids. Acidic stress results not only from acid rains, acidic polluters and acidic waste water, but it might also result from increasing stock density under aquaculture circumstances. It is already known that acidic water has negative effects on fish. The present study investigated the effect of acidic stress on haematological, immunological and serum biochemical variables of the diploid and triploid rainbow trout, Oncorhynchus mykiss. Acidic stress was achieved by exposing the sampled fish to acidic water (pH 5.2) for 4 days. Results showed that the red blood cell count, hematocrit, hemoglobin concentration, mean corpuscular volume, mean corpuscular Hb, and mean corpuscular Hb concentration, glucose, total protein, albumin, globulin, triglyceride and cholesterol were not significantly affected by acidic stress in diploid and triploid fish. However, an acidic stress caused an increase in the NBT, lysozyme and myeloperoxidase activity of diploid fish. This study results indicate that the some non-specific immune functions of triploid rainbow trout did not affect by acidic water.