Transcription of the cruxrhodopsin (cR) gene in extremely halophilic archaeon Haloarcula japonica strain TR-1 was investigated using Northern analysis to quantify message level. In the cell cultures growing in the dark, cR transcript level remained very low. In contrast, exposure of the cell cultures to light stimulated transcription of the cR gene. In addition, cR gene transcription was also induced when Ha. japonica was grown under high oxygen tension and then shifted to low oxygen tension in the dark. These results suggested that transcription of the cR gene is regulated by high light intensity and low oxygen tension.