Abstract: The present research work aimed at the effect of five photoperiod regimes on the health status of rainbow trout (Oncorhynchus mykiss) assessed through hematological components. The results revealed no significant (P<0.05) changes in haemoglobin and RBC count as compared to normal in all the treatment groups. However significant (P<0.05) changes were recorded in immune component of fishes subjected to light duration ranging from 24h to 12 hours. Differential blood count, neutrophil and monocyte count was reduced in LD24 to LD12, as compared to control. However, thrombocytes showed higher titres in stress condition of fishes, subjected to longer light durations.