Summary 1. The growing or larval stages of flies are dependent upon certain accessory growth factors which must be ingested with the food. 2. These accessory growth factors are obtained by the larvae from bacteria or yeasts, as well as from higher plant and animal tissues. 3. When rearing flies on media sterilized at high temperatures, the necessary growth factors for the larvae are destroyed, but can be replaced by contaminating the media with living bacteria or yeasts, by adding large quantities of dead bacteria or yeasts killed at low heat, or by adding fresh sterile animal tissue extracts or fresh, sterile plant juices. 4. Non-pathogenic bacteria, when ingested by adult flies shorten their life span and reduce the number of ovipositions. 5. Microörganisms and their activities are not absolutely essential to the normal growth, development, and longevity of the flies investigated. 6. Microörganisms are present in wild flies, especially in the larvae, because owing to their habits the ingestion of bacteria, yeasts, fungi, etc., cannot be avoided. 7. Microörganisms may be one of the principal sources for the accessory growth factors of some larval flies found breeding in certain types of media in a state of nature, but this assumption must not be regarded as a proved fact.