Fiber loss fluctuations due to manual handling are sometimes inevitable when it comes to touching cable connection points during maintenance and operational activities. These fluctuations cause momentary input optical power changes and give raise to bit errors in receivers. This phenomenon is a serious problem for fiber optic subscriber systems in which cable construction and maintenance frequently take place. Measures to counter this degradation have to be considered to realize stable transmission characteristics. As a result of this consideration, the loss fluctuation tolerance of digital optical receivers has been investigated for fiber optic subscriber transmission systems. From the viewpoint of design and adjustment, the most effective method to solve the fluctuation problem is to adopt d.c. balanced transmission line coding. A receiver having an optimum low cutoff frequency for this type of coding performs well against fiber loss fluctuations without adjustment for special operational conditions.