The effects of the polydnaviruses of the ichneumonid Campoletis sonorensis and the braconid Cardiochiles nigriceps on host development are reviewed briefly. It appeared that both polydnaviruses interfered with the prothoracic glands of the final-instar larvae of the host, Heliothis virescens, and suppressed pupation. The prothoracic glands were responsive to the polydnavirus for a short period and the response was influenced by the dose injected. Differences existed between the action of these viruses: Campoletis sonorensis polydnavirus caused prothoracic gland cell degeneration while the combination of Cardiochiles nigriceps polydnavirus and venom reduced the response of cells to prothoracicotropic hormone. none