Abstract: Apparatus, and an accompanying method, for accurately detecting bit-error densities in a serial bit stream and particularly relatively low bit-error densities. Specifically, detected bit-errors are synchronized to a bit clock using a error retiming circuit (130). Subsequently, the bit-errors are counted using a first counter (165) during a pre-defined measurement interval, equivalent to a specified number of bit clock cycles. A second counter (205) establishes the measurement interval by counting the specified number of bit clock pulses. Whenever the first counter attains a count larger than a pre-defined count, during the measurement interval, an output (alarm) signal is generated to indicate that a pre-defined threshold error density has been exceeded. Both the first and second counters are then reset at the conclusion of the measurement interval, and so on for successive measurement intervals.