Feeding behavior of the fruitfly, Drosophila melanogaster, is examined quantitatively by determining the amount of fool intake colorimetrically. The obtained differential sensitivity (Weber fraction δl/l = 0.025) by the two-choice test is surprisingly acute, compared with the previously reported values for insects. Besides, files also choose only the most stimulative prey in three-choice tests and even a single fly, free from 'ganging up' exhibits a clear intake choice. Adaptation in the test organ was experimentally proved not to play a leading role in ingestion choice. The present experimental results support an important role of the central nervous system in choice behavior. Although deprivation significantly enhances the amount of ingestion, it does not affect the choice itself under the present conditions, suggesting that the choice centre exists separately from the ingestion center. Discrimination sensitivity depends considerably on the food distribution and taste intensities paired.