Abstract: A frame synchronizer is disclosed for searching for the framing bits of an N bit frame in a stream of binary signals. Means are provided to compare, bit by bit, the signals in different bit positions of two adjacent frames. When only a single bit position indicates an alternating pattern of binary signals, it is considered the framing bit and a framing pulse is provided in that bit position. Apparatus is also provided to detect loss of synchronization including a pair of counters, one counting framing pulses and the other counting alternating signals in that bit position. To reduce the likelihood of false out-of-synchronization signals, the second of these counters can be pre-set. Only when the count of the first counter exceeds the number of signals counted by the second counter by this pre-set quantity will out-of-synchronization be declared.