The toxic effect of NaF (1/10 of LC50 & 1/5 of LC50) was investigated after calculating the LC50 value, on gill tissues of the fish Mystus montanus after 90 days. Possible ameliorative effect by Probiotic bacterium Lactobacillus rhamnosus was investigated after 60 days. Fluoride induced hyperplasia, desquamation of the epithelial cells, complete fusion of secondary gill lamellae and congestion of blood sinuses were the significant histopathological lesions observed in the gill tissues of Mystus montanus. Fish fed with Probiotic bacterium L. rhamnosus ameliorated the lesions of gill tissue. The recovery was more pronounced in the fish group fed diet supplemented with Probiotic bacterium L. rhamnosus. However, slow recovery was observed in depuration group of fish.