An 8-week feeding experiment was conducted to develop a semipurified diet for the red sea urchin Pseudocentrotus depressus. Casein, casein hydrolysate and crystalline amino acids were tested in an attempt to find an adequate source amino acid for the urchin diet. In addition, two binders, sodium alginate and Curdlan, were compared. The growth in test diameter and body weight, and feed efficiency became higher as the amino acid chain length increased. Urchins fed a diet containing casein showed the highest values of body protein content, protein efficiency ratio and apparent net protein utilization. The energy availability and digestibility of the diet having crystalline amino acids showed the lowest values. The diet with casein and sodium alginate showed a higher growth rate, lower feed efficiency and lower protein retention than the diet with casein and Curdlan. These results indicated that casein is the most suitable of the three amino acids sources tested in this experiment. The choice of binder can be changed depending on the objective, that is, growth or diet efficiency.