New information concerning spawning and reproduction of the fathead minnow (Pimephales promelas Rafinesque) from observations in Colorado, and a review of important findings of previous works is presented in this paper. Potential for egg production by mature. Age I females is between 1,000 and 10,000 eggs per breeding season. On the basis of this information and the known polygynous mating behavior, we estimate that 19,200 adult fathead minnows in a ratio of five females per male should be stocked per surface acre to yield a theoretical maximum production of 1.5 million fingerlings per surface acre under intensive-culture conditions. Spawning usually starts in May with exact timing of initial spawning apparently controlled by day length but influenced by many factors. In one fathead minnow population in Colorado water temperature at the time of the first observed spawns was 1.6 C lower than had been reported by previous investigators.