The average magnitude of earthquakes produced along the Parkfeld d segment of the San Andreas fault was decreased significantly between late 1986 and late 1988 along with the seismic quiescence [Wyss et al., 1990a>. The change corresponds to an increase of the b-value from 1.1 to 1.4. During the last year (1989) however, the average magnitude produced was approximately normal again. This same phenomenon had been identified before several other mainshocks. Therefore it may be interpreted as a precursor to the predicted Parkfield mainshock. ¿American Geophysical Union 1990