A summary is given of the types of signal which might be perceived by the mole in itsunderground life, viz. sound, seismic earth movement or vibration, light, infrared radiation and other electromagnetic disturbances, and odours. Some guidance is provided regarding suitable physical receptors to use in the investigation of these various signals. Satisfactory receptors are available for light, infrared radiation and the other classes of electromagnetic disturbance. For sound, the presently available receptors are of inadequate sensitivity. Seismic earth movements and odours are almost unexplored and their detection at an appropriate level of sensitivity is a suitable subject for future research.