The present study investigated the relationship between water quality and fish species diversity in the Kodaiyar Hill streams of the Western Ghats, Kanniyakumari District, Tamil Nadu, India. Three sampling stations were selected and monitored monthly from February to July 2005. Physicochemical parameters including temperature, dissolved oxygen, salinity, and pH were analyzed following standard procedures recommended by the American Public Health Association. Fish samples were collected using seine nets and identified using standard taxonomic keys. Species diversity was assessed using Shannon–Wiener diversity index, Simpson's dominance index, Hill's diversity numbers, Margalef's richness index, Menhinick's index, McIntosh index, Berger–Parker dominance index, and Pielou's evenness index. 

A total of seven fish species were recorded from the three sampling stations. Among them, Rasbora daniconius and Danio aequipinnatus were the dominant species throughout the study period. Dissolved oxygen concentrations ranged from 6.4 to 7.5 mL L⁻¹, indicating favourable environmental conditions. Shannon–Wiener diversity index values ranged from 1.18 to 1.42 at Station I, 1.28 to 1.65 at Station II, and 0.54 to 1.39 at Station III. Station II exhibited the highest diversity and evenness, whereas Station III showed lower diversity during April, suggesting localized environmental disturbances. The findings indicate that relatively undisturbed habitats support higher fish diversity and demonstrate that fish diversity indices are effective tools for assessing ecological health and water quality in hill stream ecosystems.