The development of an aquatic toxicity index and its application is described. In this index the protection of aquatic life is always referred to in terms of toxic effects of different water quality variables to fish, as health indicators of the aquatic ecosystem. The final index score is produced by means of standard additive techniques as well as by using the water quality variable giving the lowest index score (minimum operator). The minimum operator is employed in order not to conceal important water quality information. The aquatic toxicity index development has been linked to toxicological data, international water quality standards and South African guidelines. The index provides valuable information concerning toxic effects of a specific variable on fish should the threshold level for normal maintanence of aquatic life be exceeded. This index is intended as an aid in the interpretation of water quality information in order to facilitate management decisions.