The mean focal depth of earthquakes in the northern and southern parts of the Parkfield segment of the San Andreas fault was greater than the long term average by 1 km during the last 4 years approximately. It is proposed that this is a precursor to the next characteristic Parkfield earthquake, because the same phenomenon occurred before at least three other mainshocks along the San Andreas fault. In the northern segment the increase in mean depth is mostly due to reduction of shallow activity, in the southern segment deeper than usual activity seems to have increased. Within about 8 km from the 1966 epicenter no significant increase of mean depth is observed, but seismic quiescence [Wyss et al., 1990a> and a decrease in deformation rate [Wyss et al., 1990b> was documented. These observations are interpreted to suggest that the precursory process at Parkfield varies along strike, and that the next rupture may extend further north than the last one. ¿ American Geophysical Union 1991