This review describes the upstream-directed movement in the small parasitic bacterium Mycoplasma. Many Mycoplasma species exhibit gliding motility, a form of biological motion over surfaces without the aid of general surface appendages such as flagella. The gliding motility is characterized by a constant unidirectional movement without changes in direction or backward motion. Unlike flagellated bacteria, Mycoplasma lacks the general chemotactic signaling system to control their moving direction. Therefore, the physiological role of directionless travel in Mycoplasma gliding remains unclear. Recently, high-precision measurements under an optical microscope have revealed that three species of Mycoplasma exhibited rheotaxis, that is, the direction of gliding motility is lead upstream by the water flow. This intriguing response appears to be optimized for the flow patterns encountered at host surfaces. This review provides a comprehensive overview of the morphology, behavior, and habitat of Mycoplasma gliding, and discusses the possibility that the rheotaxis is ubiquitous among them.