A geodetic trilateration network comprising 54 baselines was established around the northern part of the Imperial fault beginning 12 days after the Imperial Valley earthquake of 15 October 1979, and the line lengths were repeatedly observed during the following 110 days. During the observational period, there was an average of about 10 cm of dextral postseismic fault slip. Geodetic data in conjunction with alignment-array measurements indicate that the postseismic slip U at time t following the main shock is well described by U (t) = (10.96 ± 0.12) cm log(t/1.75 day + 1), a result that implies that it will be approximately 9 to 10 yr before the rate of postseismic displacement diminishes to the ambient rate of creep of 0.5 cm/a. The postseismic strain changes are localized to the east side of the Imperial fault in the southern part of the Mesquite Lake basin, where significant north-south extension occurred at an average rate of 2.7 ± 0.3 × 10−5/a during the observational period. These changes are compatible with dextral fault slip within the top 5 km of the fault.