The possiblity of suppressing the growth of Nocardia (Rhodococcus) rhodnii symbiont in the gut of Rhodnius prolixus by feeding the bugs on rabbits immunized against the bacterium, was investigated. Rabbits immunized with symbionts isolated from adult bugs and grown in vitro developed high antisymbiont titres (1,024-2,048) and showed no ill side effects. The log-phase growth of the symbionts was retarded in the gut of R. prolixus larvae fed on immunized rabbits. Both growth rate and maximal number of symbionts were lower in groups receiving treatment. Rate of development of bugs reared on immunized rabbits was slowed and their success in moulting was reduced.