Notes: Summary 1. Two methods were used to attempt to infect sterile Stegobium larvae: 1. Smearing the eggs with the foreign yeasts and the taking up of the same by the larvae at the time of hatching; 2. mixing the yeasts in the fresh as well as dry state with the food of the young larvae. Only the second way proved successful. Of the 15 yeasts in pure culture (symbiotic yeasts of Cerambycidae and different culture yeasts) a successful infection was obtained only with dried Torulopsis utilis. Larvae from sterile eggs, which were smeared with yeast suspensions, died before the first molting on a diet of wheat grit and flour without the addition of yeast extract. 2. The content of vitamins of the B-complex, using Tribolium confusum as the test object, was determined in the yeasts, in their substrates both before and after inoculation, in the normal diet (wheat grit and flour), and in the yeast extract. 3. The addition of fresh yeasts to the foòd of sterile larvae so supplemented the diet of wheat grit and flour that all larvae developed into adults. In no case did an infection of the blind sacs take place. 4. After adding fodder-yeast (T. utilis dried) to the diet of the sterile larvae, there followed an infection not only of the blind sac epithelium but also of all epithelial cells of the mid-gut. The addition of other dried yeasts to the food of sterile larvae supplemented the diet to such an extent that all larvae developed into adults, without however becoming infected. 5. Sterile Stegobium larvae, to which species specific symbionts and dried T. utilis were offered in the food at the same time, took up only their normal symbionts into the mycetocytes of the blind sacs; an infection of the slender epithelial cells of the blind sacs and the epithelium of the mid-gut with T. utilis did not take place. Normally infected Stegobium larvae, to which dried T. utilis was offered in the food, were not infected by this yeast. 6. Corresponding to the varying content of vitamins of the B-complex in the yeasts, which were added in the dry state as 5% of the food, different developmental times were obtained: from 32 to 38 days with Torulopsis albida to from 60 to 70 days with Candida mesenterica. Larvae on diets with T. utilis had a growth period of from 34 to 40 days. 7. Those larvae, which were kept for the first two weeks of life on diets with dried T. utilis and were then transferred to wheat grit and flour (with nothing added), completed their development only after 51 to 56 days. All of these larvae had become infectet with the foreign yeast during the first two weeks. 8. In the adult stage the content of pantothenic acid, riboflavin and nicotinic acid in the diet exerts no apparent effects. The life span of all adults (those normally infected as well as those free of symbionts) is the same in wheat grit and flour both with and without yeast extract. 9. The vitamin content of the food of the parental generation has an effect on the developmental time of the F1-generation. If the parents receive T. utilis in their larval diets, all F1-larvae, hatching from the sterile eggs, develop to adults in wheat grit and flour (without enrichment). However the time of development of this F1-generation is noticeably extended. When Candida mesenterica, Endomyces magnusii, Saccharomyces cerevisiae, S. pastorianus, Torulopsis albida, T. famata and Zygosaccharomyces mayor were added to the food of the parental generation, then only a small percent of the sterile F1-offspring reached metamorphosis. The other' yeasts tested had no influence on the development of the sterile F1-larvae; all of which died on diets of wheat grit and flour. 10. The life span of the F1-adults, whose parents were raised on diets with T. utilis, was 12 to 19 days as compared with 2 to 12 days for adults whose parental generation was fed diets with the above mentioned yeasts. 11. All F2-larvae from these experiments died before the first molting, in so far as the F1-generation was raised on wheat grit and fluor. 12. The histological investigation of the larvae artificially infected with T. utilis showed that all epithelial cells of the blind sacs of the mid-gut and of the entire mid-gut itself were infected. Therefore that barrier, which is impenetrable to the normal symbionts, is not set up against these foreign yeasts. 13. Those cells, which always loose their ciliated lining in an infection with their species specific symbionts, retain it in the infection with T. utilis. 14. During the pupal rest the newly formed epithelium of the mid-gut is not reinfected by T. utilis. The adults are therefore usually sterile. In a few cases the yeast used (T. utilis) could be cultured from shredded blind sacs and guts. The egg laying apparatus of the female adults is never infected. Also examination of the mid-guts of the F1-generation, which was raised on fresh wheat grit and flour, confirmed the fact that no transference of the foreign yeast to the offspring had taken place.