Cyprinus carpio (C. carpio) fish were exposed to chronic sublethal toxicity (one-tenth of the LC₅₀/96 hours - 0.071 ppm/l) of phorate (CSTP) for 1, 7, 15 and 30 days and the effect of CSTP on gill histopathology of the fish was investigated. On exposure for a period of 1 day to CSTP, mild degenerative changes were observed in the gill of the fish. After 7 days of exposure, a further damage in the structure of the gill was observed and it was progressed up to day 30. The primary and secondary lamellae of the gill showed epithelial hyperplasia and hypertrophy. Atrophy and swelling at the base with desquamation of the hyperplasic epithelia in the secondary gill lamellae was observed. Significant hemorrhagic condition was also noticed in the gill. The findings of the present investigation suggest that the frequency of pathological changes increases with increasing the exposure time to CSTP.