1. Imagos of Drosophila raised at temperatures of from 12-28.5 degrees C. when placed at any temperature from 15-32.5 degrees C. produce eggs which develop normally at these temperatures. 2. Imagos raised at temperatures of from 29-32.5 degrees and then kept permanently within these temperatures produce eggs which do not develop. 3. Imagos raised at from 28.5-32.5 degrees C. and then placed at temperatures of from 12-25 degrees C. produce eggs which develop normally. 4. Imagos raised at from 28.5-32.5 degrees C. placed at 15-25 degrees C. for 24 hours or longer and then put back into a temperature of from 28.5-32.5 degrees C., produce eggs which will develop at the latter temperature. 5. There is no evidence of any hereditary adaptation to higher temperatures.