In members of the Archaea positive and negative phototactic responses are mediated via retinal-containing sensory rhodopsin photoreceptors, according to a mechanism that is similar to the mechanism of enterobacterial chemotaxis. In Bacteria the situation is less well resolved, even though the accumulation of photosynthetic bacteria in a light spot is one of the most extensively studied tactic responses of prokaryotes. Only recently, however, has it been reported that in this family of organisms (i.e. in the purple- or proteobacteria) another type of phototactic response occurs: blue light, of physiological intensities, evokes a repellent response. The photoreceptor that presumably mediates this response is the Photoactive Yellow Protein (PYP), a member of the xanthopsins. This family of photoreceptors consists of 4-hydroxy-cinnamate containing proteins, for which rich detail concerning structure and function is available. In this contribution we will review the structure and function of PYP, and the initial molecular genetic studies aimed to further characterize the signal transduction chain responsible for the photoresponses mediated through PYP.