Exposure of Cyprinus carpio (common carp) to sublethal concentrations of the organophosphate insecticide chlorpyrifos produces measurable haematological and biological perturbations. Reported responses across laboratory studies include significant alterations in red blood cell (RBC) count, haemoglobin (Hb) concentration, packed cell volume (PCV), and white blood cell (WBC) profiles, together with suppression of acetylcholinesterase (AChE), increased oxidative stress (lipid peroxidation), changes in antioxidant enzymes, and histopathological lesions in gill, liver and kidney. The direction and magnitude of blood- and tissue-level changes are exposure- and time-dependent, and frequently correlate with AChE inhibition and markers of oxidative injury. These integrated endpoints make haematology a sensitive tool for sublethal chlorpyrifos effects and for ecological risk assessment in freshwater systems.