Apistogramma agassizii is a dwarf cichlid species endemic of the Amazon basin, about which there is little biological and ecological information in their natural habitat, despite their ornamental importance. This study evaluated the populacional structure A. agassizii at Amana Sustainable Development Reserve, Middle Solimões basin, Amazonas, Brazil. Positive sexual dimorphism found is probably related to the reproductive behavior of the species. Some management suggestions are offered to support the sustainable use of the species for the studied sites.