This study was conducted to examine the nutritional value of eight algal diets for two species of abalone,Haliotis tuberculataandHaliotis discus hannai, by measuring biochemical composition of the algae and relating this to feeding rate, growth and biochemical composition of the animals. Nutritional value of algal diets can be divided into three categories for each species of abalone. ForH. tuberculatathe best performance was on the mixed diet andPalmaria palmataintermediate wasAlaria esculenta, Ulva lactucaandLaminaria digitata, and lowest growth was onLaminaria saccharinaandChondrus crispus.ForH. discus hannai, best performance was onA. esculenta, P. palmataand the mixed diet; intermediate was onL. saccharinaandL. digitataand lowest was onU. lactuca.It is generally accepted that high “balanced” levels of protein (>15%), lipid (3–5%) and carbohydrate (20–30%), with no detrimental substances in natural algae are essential for optimal growth performance of these abalone. The fact thatA. esculenta, L. saccharinaandU. lactucahad different dietary values for the two abalone species indicates specific nutritive requirements and/or digestive physiology. Overall,H. tuberculatagrew faster, had higher food conversion efficiencies and muscle yield thanH. discus hannai.Generally abalone fed on the highest category diets, had higher muscle yields and levels of protein, visceral lipids and muscle carbohydrate. Viscera and foot muscle are reservoirs for lipid and carbohydrate, respectively. The effect of algal diet on sexual maturation is similar to that on somatic growth.