Reports a survey of the exposure of staff in the UK Electricity Supply Industry to power-frequency magnetic fields. Two hundred and fifty-eight staff from a variety of jobs and locations wore a monitor for 1 week each. The results form a substantial body of data which adds significantly to the understanding of occupational exposures. The results show that fields encountered in shops, offices and distribution sites were roughly half those in power stations, which in turn were half those in transmission sites. Office workers based on transmission sites experienced higher fields (geometric mean of individual time-weighted average fields: 0.48 mu T) than those at headquarters offices (0.18 mu T). In power stations, electrical workers experienced higher fields (0.46 mu T) than mechanical workers (0.25 mu T). Amongst transmission and distribution staff, the highest fields (1.16 mu T) were experienced by transmission substation attendants and the lowest fields (0.17 mu T) by staff working predominantly in domestic environments.