Fluoride has already been identified as a potential toxic contaminant in the environment causing ailments affecting different systems of body [1]. Toxic manifestation of this element in human beings and other experimental animals is widely documented but very few studies have been conducted on its toxicosis on fish fauna. In this study toxic manifestation of the environmental stress owing to the sub-acute to sub-chronic exposure of fluoride on haematological parameters were evaluated on the chosen fish species Catla catla. The exposure duration was 30, 60, 90 and 120 days. The exposure of excess fluoride concentration in water for long-term duration caused adverse alteration on various parameters of haematopoietic system of the fish. Decrease in haemoglobin, RBC count, MCV, MCH, MCHC and WBC was recorded, and its magnitude was time dependent. These toxic effects are the matter of concern for aquaculture and indicative of the physiological abnormality caused by this environmental contaminant.