Optical frequency multiplex systems where several signal flows are carried over a single fiber are subjected to crosstalk. This crosstalk causes an unwanted increase of the bit error rate (BER) in the wanted channel and is encountered in both unidirectional and bidirectional traffic. Measurements 'made to determine the crosstalk power ratio required for a given BER are outlined for 678-Mbit/s fiber optic transmission systems. A description is also given of bidirectional transmission tests on singlemode fibers.