1. Chemically pure thyroxin solution was used to study the effect on the metamorphic rate, fecundity, sex ratio, size and crossing-over ofDrosophila melanogaster. 2. The data obtained show thatDrosophila does not respond to thyroxin in the same way as do the Vertebrates. 3. Some small alteration (e. g. in the egg-laying period) was due to the secondary effect of thyroxin; i. e. the altered pH of the food. 4. The negative results obtained byDOBKIEWICZ with dried thyroid gland were confirmed when pure thyroxin was used.