A telluric array has been designed to monitor long-period changes of resistivity in Parkfield prior to the next characteristic earthquake there. The array straddles the San Andreas fault and utilizes dipoles which range from 5 to 18 km long. A method of data analysis using robust processing in the time domain and singular value decomposition yield estimates of telluric coefficients which are stable to within 1%. The array has detected several fluctuations above the noise level of the measurements. Most of these fluctuations are uncorrelated with bursts of seismic activity, changes in water levels, anomalous strain and creep, or magnetic storms. However, the largest fluctuations in 2 years coincided with a tensor strain anomaly, and both of these preceded a M=3.7 earthquake by approximately 1 month. This is the largest earthquake to occur in Parkfield since the array was installed. ¿American Geophysical Union 1991