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We have developed a cassette of the bacteriophage lambda (λ) cI857 temperature–sensitive repressor gene from which the functional PRpromoter has been deleted. This cassette was placed into an expression vector that uses the PLpromoter for expression of the herpes simplex virus type 1 (HSV–1) glycoprotein D gene (gD–1) in E. coli. Control of the gD–1 production from this plasmid was compared to an expression vector lacking the cI857 cassette but regulated by an E. coli (cI857) lysogen. The lysogen allowed expression of the gD–1 gene at 37°C, whereas cells containing the cI857 cassette maintained repression of gD–1. Both systems were fully induced at 42°C. The ability to grow cells at 37°C while maintaining repression of transcription from PLshould be advantageous for large scale fermentations.