No precursory variations of either resistivity or self-potential were seen to precede an earthquake of magnitude 4.0 which occurred in December 1977 on the San Andreas Fault in central California. This result is in contrast with an earlier result, in which a variation of about 10 to 15% was seen to precede an earthquake of similar magnitude and location which occurred in June 1973. As the present accuracy of the resistivity measurement is about ¿2%, this result suggests either that the relationship between earth strain and resistivity was different for the two events, or that the earlier result was erroneous and that the amplitude of any resistivity variation preceding an earthquake of magnitude 4 in this area is less than 2%. This result implies that much better measurement accuracy will be required to detect any variations preceding earthquakes of magnitude 4 or less in this area.