In motor-respiratory coordination, people typically maintain frequency ratios that are from lower levels of a mathematical structure known as the Farey tree. A hypothesized mechanism for motor-respiratory coordination is the visceral piston (Bramble & Carrier, 1983)-mechanical loading of the lungs due to footfalls, for example, imposes a rhythm on breathing. The occurrence of motor-respiratory coordination in exercises in which there is no visceral piston (e.g., bicycling and wheelchair propulsion) implies that there are other mechanisms. We examined whether there is a perceptual basis for motor-respiratory coordination. In Experiment 1, participants viewed simulated ratios side by side and, in a forced-choice paradigm, judged whether they were the same or different. In Experiment 2, participants performed ratios using feedback displays in which different ratios looked either the same or different. Lower level ratios were less likely than higher level ratios to be confused with other ratios. Ratios that could be distinguished perceptually were performed more accurately and less variably than ratios that appeared the same.