In 1915 Morgan descr. a dominant, sex-linked mutation "Abnormal Abdomen" of Drosophila melanogaster. This gene varied its expression under different environmental conditions, it shows best manifestation only in moist food and the flies hatching first are the most abnormal. The author worked with a recessive, autosomal mutation a-III (abnormal abdomen) of D. m. which pheno-typically looks similar to Morgan''s "Abnormal Abdomen" but needs just the opposite environmental condition for its best manifestation. Penetrance and expressivity are highest under the conditions lack of food and dry food and increases towards the end of the hatching-period.