A twist detecting device for use in a detwisting apparatus for a roped web of knit or woven fabric in a fabric processing system comprising a pair of sensing drums each having a plurality of axial grooves formed on at least a portion of the circumferential surface thereof and spaced apart from each other so that said roped web passes between said drums in a direction generally parallel with said grooves; means for supporting said drums rotatably about their respective axes so that as said twist in said rope is tightened, engagement of said twisted rope with said grooves on either one or both of said drums causes simultaneous rotation of said drums in the same direction, the direction of said rotation depending on the direction of said twist; and means for detecting said direction of rotation to produce a corresponding electrical signal which actuates a detwister so as to decrease or remove said twist in said roped web.