Abstract Microinjection of 1- 14 C- β -alanine into immobile larvae at spiracle eversion confirmed that this substance is utilized during tanning of the pupal case in Drosophila melanogaster . Two mutants, ebony and black , which fail to tan the pupal case were shown to be defective in the metabolism of β -alanine. Pharate pupae of ebony are able to manufacture the compound but unable to utilize it. Pharate pupae of black are unable to manufacture the substance, but can utilize β -alanine if it is supplied to them during the larval stage. The variation in the concentration of β -alanine throughout development was determined and a comparison between ebony and normal was made. Beta-alanine is normally manufactured at puparium formation and adult eclosion. Pharate pupae and young adults of ebony have excess amounts of β -alanine, because of the apparent inability of this mutant to utilize the substance. However, mature ebony adults have normal levels of β -alanine. Therefore, it is improbable that the abnormalities observed in thevisual behaviour of this mutant can be ascribed to excess β -alanine.