In the bacterial chemotaxis network, receptor clusters process input(1-3), and flagellar motors generate output(4). Receptor and motor complexes are coupled by the diffusible protein CheY-P. Receptor output (the steady-state concentration of CheY-P) varies from cell to cell(5). However, the motor is ultrasensitive, with a narrow operating range of CheY-P concentrations(6). How the match between receptor output and motor input might be optimized is unclear. Here we show that the motor can shift its operating range by changing its composition. The number of FliM subunits in the C-ring increases in response to a decrement in the concentration of CheY-P, increasing motor sensitivity. This shift in sensitivity explains the slow partial adaptation observed in mutants that lack the receptor methyltransferase and methylesterase(7,8) and why motors show signal-dependent FliM turnover(9). Adaptive remodelling is likely to be a common feature in the operation of many molecular machines.