Abstract: A well data telemetry system for transmitting data along at least a part of a well bore containing tubing of magnetically permeable material comprises a transmitting coil located to transmit a magnetic flux signal along the tubing and/or the surrounding geological formations, modulation circuitry connected to the coil for modulating the magnetic flux signal in response to a data signal, and an a.c. magnetometer located to detect and respond to the magnetic flux signal at a position spaced from the transmitting coil. By using an a.c. magnetometer instead of a conventional receiver coil to detect and respond to the magnetic flux signal, the system is capable of responding to the low levels of signal which are likely to be received in practice. Furthermore, the response of the a.c. magnetometer is independent of frequency so that the system may be used at low frequencies as well as high frequencies and demodulation of the transmitted signal can be effected without difficulty.