Third- or fourth-stage nymphs of the large milkweed bug Oncopeltus fasciatus (Dallas) were maintained for 2–5 months on a chemically defined diet. Four or five per cent of either sucrose, glucose or melezitose gave better growth than 11 other sugars tested. The insect required the 10 amino acids usually recognized as essential for growth in other insect species. The dietary concentration of each essential amino acid was varied over a wide range of 6–12 or more times without affecting growth in a significant manner. Growth improved as the total amino acid concentration was increased from 0 to 1.2, 1.8, 2.4 and 3.6%, but a further increase to 4.8% did not result in an improved growth. Fifth-stage nymphs maintained on chemical diets moulted to the adult stage and laid eggs for several months; the average number of eggs per female was 62 on the 1.2%, 143 on the 2.4%, 131 on the 3.6%, and 69 on the 4.8% amino acid diets. No eggs were laid on a diet lacking amino acids. Growth and survival of nymphs reared singly on the diet were reduced as compared to that of nymphs reared in small groups. A quarter to half reduction in the concentration of the two major dietary salts, as well as a reduction in the pH from 7.0 to 6.0 or 5.0, or an increase to 8.0 improved growth.