Abstract
1. Earlier views of the nature of the association between Rhodnius and its symbiotic bacteria are confirmed. 2. The bacterium is described as Nocardia rhodnii in accordance with more recent views of the classification of the Gram-positive filamentous bacteria. 3. Experimental evidence is provided to show that the symbiotic bacterium is essential to Rhodnius because of its function in supplying certain B group vitamins, in which the normal blood diet of the host is deficient.