Random transcription fusion delivery, with bacterial luciferase genes as reporter, was performed in the psychrotrophic bacterium Pseudomonas fluorescens. Direct screening on plates of the insertions allowed the isolation of fusions into thermoregulated genes with good accuracy, either in a library of insertion fusions, or after genetic transfer of a putative regulatory mutation. Using this method, it was shown that in Ps. fluorescens, nearly 40% of the genes are thermoregulated and belong to at least three classes according to the maximal temperature of expression of the fused genes. This is more than had been estimated by a previous method, and demonstrates the importance of thermoregulation in psychrotrophic bacteria. As this reporter is the first to be used for direct screening for genes regulated by temperature, it should be of great value in the study of mechanisms involved in adaptation to this environmental factor.