Contents: 1. Introduction. 2. Study Area. 3. Spatio-Temporal Variation in Population Abundance, Demographic Structure and Biomass of the Fish. 4. Food and Feeding Habits of the Fish. 5. Reproductive Biology and growth Studies on Gambusia Holbrooki. 6. Experiment to Study the Impact of Gambusia Holbrooki on Plankton Community. 7. Experiments on the Nutrients Excretion by the Fish. 8. Relationships between the density of Gambusia and Various Biotic and Abiotic Factors of the Lake Nainital. 9. Experiments on Control of G. Holbrooki by Different Methods and Some Suggestions for Biomanipulation. References.