“Signal imitation” is the term used in telephony to describe the operation by speech currents of voice-frequency signalling equipment. There are means by which signal imitation can be reduced to any desired extent, and eliminated where necessary, at some sacrifice of simplicity of the equipment and of signalling speed. The paper is concerned with analysing the means of reducing signal imitation so that a judicious choice of signals may be made. It is based largely on work undertaken in connection with the standardization by the C.C.I.F. (Comité Consultatif International Téléphonique) of a voice-frequency signalling system for international telephone circuits.The work was primarily directed towards discovering the signalling frequencies least liable to signal imitation. It was found that these frequencies depended on the origin of the speech currents, commercial microphones and telephone circuits giving results very different from high-quality recordings. The effect of language is small compared with that of other variables. Mainly on the evidence of these results, recommendations were made by the 8th C.R. of the C.C.I.F. to use frequencies between 2000 and 2500 c/s for international signalling in Europe.The work was then extended to collect data on the influence on signal imitation of factors other than frequency and language.