Seven weeks prior to he M=5.1 Hollister, Calif., Thanksgiving Day earthquake of 28 November, 1974, and anomalous magnetic variation was observed at one of the magnetometers of the USGS array. The anomaly lasted for about three weeks. Recently developed methods or reducing noise on magnetic records reveal that anomalous magnetic changes occurred at about the same time at three of the six stations analysed. Such changes have not been seen either previously or subsequently. The largest variation occurred at the two stations closest to the earthquake, but a change also occurred at a station 44 km to the south.