THE literature on symbiosis, as summarized in Buchner's1 latest book, refers almost entirely to histological and cytological findings. Little work seems to have been done on the isolation of the symbiotes, and less still on their physiological role in insect metabolism. Koch2 was the first to prove that the symbiote of Sitodrepa panicea provides a growth-promoting factor or vitamin for this insect. With regard to Cicadella viridis, I have published two3,4 communications showing that the insect contains two bacteria : one on culturing produces a greenish-yellow pigment, identical with the colour of the insect, and the other -carotene which is apparent at least on the legs of the males. In the female, -carotene is reduced to the colourless vitamin A, which is needed by the insect.