Streptomycetes are prokaryotic microorganisms that exhibit a complex, mycelial fungus-like cycle of morphological differentiation. Development involves at least two spatially separated types of cells: the branching hyphae of the substrate mycelium, which penetrate the stratum upon which the colony feeds, and the upwardly protruding hyphae of the aerial mycelium, which undergo metamorphosis into spores. The luciferase-encoding luxA and luxB operon of the luminescent marine bacterium Vibrio harveyi was used as a promoter probe to visualize gene expression in differentiating colonies of Streptomyces coelicolor. Promoters for developmental genes of several kinds gave distinctive temporal and spatial patterns of light emission.