The numerical and biomass structures of the fish biofacies from a small stream were studied by means of diversity indices. Biweekly samples taken over a year's period showed temporal fluctuations within a narrow range of variation. Seasonal changes in species composition had very little effect on the overall numerical structure of the fish biofacies, as species richness and diversity indices based on numbers of individuals showed stability throughout the year. Contrastingly, the number of species per collection and diversity indices based on biomass exhibited seasonal variations in response to seasonal changes. In total, the numerical structure of the fish biofacies was controlled by the persistent dominance of a few resident species.