The paper attempts to study the input-output relationship in fish farming in India through the estimation of production function and the derivation of the optimum input combination. The data used belong to a cross-section of 45 districts, covering 12 states spread all over the country. These were collected through a survey of the selected 649 ponds, about 15 ponds from each of the 45 districts. The study reveals that (a) fish farming is subject to constant returns to scale, (b) production technology is far from the optimum-in particular, organic fertilizer is excessively applied while inorganic fertilizer is scarcely used, (c) application of the optimum production technique would reduce the production cost by about 33% and raise the profit rate by about three times at the current yield rate of 828 kg./hectare, and (d) if fish farming is attempted to achieve the maximum possible profit/hectare, the yield rate would multiply by about three times and the profit rate by over six times.