A method and apparatus for detecting the C-bit parity application of DS3 makes use of the relative state of the CP-bits and the P-bits in one or more M-frames alone or in combination with the state of the AIC signal. In one implementation, the invention is a detector circuit that is adapted to receive a CP-bit and a P-bit from each of a group of one or more M-frames of a DS3 service. For each M-frame in the group, the detector circuit performs a logical XOR between the CP-bit and the P-bit for that M-frame. The detector circuit further performs a logical OR between the XOR results from each M-frame in the group and outputs a C-bit parity format detect signal if the result is zero.