A deterministic mathematical model, based on the conservation of numbers, is proposed for predicting the distribution of fish eggs after they drift away from the spawning regions. In the model, the effects of egg settling, diffusion capability of the water body, river currents and river boundaries are all included. Close form solution is obtained for the governing partial differential equation. In deriving the solution, egg-flux across river banks, water surface or bottom is suppressed. Application of the model is illustrated by an example. The result should provide the biological community with a quick and easy way of predicting the location of fish eggs in a river.