The potential of bacteriophage λ as an expression vector for a large scale production of cloned‐gene proteins was evaluated in batch and continuous bioreactors using a temperature‐sensitive mutant in the cl gene, which allows a simple manipulation of temperature as a means to control the phage in the lysogenic or lytic state. A temperature switch from 32°C (or below) to 38°C (or above) forces the phage to go from the lysogenic state to the lytic state. Temperature cycling and a two‐reactor system were used for continuous cultures. For the latter the first reactor is maintained in the lysogenic state at a lower temperature to stably maintain the foreign DNA in the host cell, while the second reactor is maintained in the lytic state to force replication of the cloned‐gene and overproduction of its products. The results are promising but suggest a greater potential for a mutant which lacks the Q gene which is responsible for host cell lysis and packaging of phage particles.