A medium for the accumulation of extracellular hemolysin (300 to 1,600 hemolytic units per ml) and protease (2 to 3 proteolytic units per ml) was developed for an anaerogenic strain of Aeromonas hydrophila. In this medium, growth yields were less but levels of accumulated toxin were greater or equivalent when compared with the same responses in brain heart infusion and nutrient broths. The medium was considered to be partially defined since the conditions for maximum observed hemolysin accumulation (1,600 hemolysin units per ml) were not identified. The results showed that iron and zinc contributed to the control of the extracellular accumulation of both toxins. Whereas iron exerted an inhibitory effect, zinc stimulated the accumulation of both toxins.