ABSTRACT Strains homozygous at the amylase locus were derived from a polymorphic laboratory population of Drosophila melanogaster. The Amy 4,6 strain has higher enzyme activity than the Amy1 strain.—Maltose has the same nutritional value as starch.—The effect of starch in pure culture depends on the yeast level. At low yeast level increasing starch increases survival, at high yeast level increasing starch increases mean dry weight. The strains do not differ in survival or mean dry weight in pure culture.—In mixed cultures at 50% input of Amy 4,6 and Amy1 as larvae the percentage Amy4,6 in adults increases with increasing starch at low yeast levels, but equals input frequency at high yeast levels. No increase in percentage Amy4,6 in adults is present with increasing maltose at low yeast levels in mixed culture. The increase in percentage Amy4,6 with increasing starch must be due to selection on the amylase locus working by competition for food in the larval stage. The single locus selection coefficient is determined by the environment and can reach quite high values.—Viability selection in the presence of starch is in the direction indicated by the enzyme activities.