Two strains of aphids were subjected to various combinations of temperature and light. In one strain (collected 1923) distinctly more intermediate-winged females were produced in intermittent than in continuous light, in every combination of other conditions. Among those reared in intermittent light, more intermediates were produced at a temperature of 14° than at 24°, in every combination of other conditions. Those reared in continuous light, however, produced usually fewer intermediates at 14° than at 24°, and the differences were small. In general, winged parents produced more intermediate offspring than did wingless parents, but there was much irregularity of this relation. Greatest production of intermediates and greatest production of typical winged females did not occur at the same time; there was, indeed, a tendency for the two frequencies to be opposed to one another. Other relations in this strain were rather indefinite. In the second strain (collected 1931) more intermediates were usually produced in intermittent than in continuous light, but the differences were small. More intermediate offspring were produced by parents drawn from a stock kept at room temperature than by parents taken from stocks kept continuously at 14° or 24°, or from a stock regularly alternated daily between 14° and 24°, though again most of the differences were small. The greatest (and the only large) number of intermediates in the 1931 strain were produced by taking aphids from a stock kept at 24°, changing them for one generation to room temperature, then putting this latter generation, when adult, at 14°. In general, in this strain the greatest frequency of intermediates occurred when the frequency of typical winged aphids was medium. Other relations in the 1931 strain were not striking. If it be assumed that production of the characteristics of winged aphids be dependent on suitable concentrations of a hormone which increases in amount during development (other assumptions could be made), intermediacy or mosaicism might result from various combinations of times or periods of determination, or of levels or ranges of concentration of the stimulating hormone, or both. Ranges of stimulating concentrations are less subject to known limits than are periods of determination, and are held to be the more likely, inasmuch as strains of aphids differed greatly in their tendency to produce intermediates. Intermediacy, as contrasted with mosaicism, is favored by extended periods of determination and ranges of stimulation, especially if there is overlapping of these periods and ranges for the several structures. In part, the intermediates may replace winged aphids in the 1923 strain, and wingless ones in the 1931 strains.