A control mechanism adapted for incrementally and positively controlling the displacement of a biassed member or the like associated with a bicycle gear-change transmission between a plurality of predetermined positions for effectuating changes in gear ratios of such transmission in response to unidirectional movement of an operator control lever of the control mechanism. In addition, the control mechanism is also, preferably, provided with an "overshift" arrangement which effectively and effortlessly enables an operator to shift the bicycle sprocket chain from one sprocket to another sprocket of larger diameter. In an alternate embodiment, the control mechanism is directly mounted on and connected to a derailleur type bicycle change-gear mechanism.
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