The volcanic stratigraphy for the AhuachapLn-Chipilapa geothermal field is defined on the basis of the magnetostratigraphic results on 156 oriented samples from 33 sites. The magnetostratigraphic sequence shows that the major volcanism associated with the Concepcibn de Ataco caldera and the Cuyanausul volcano took place during the middle Brunhes chron (Quaternary). Pre-caldera activity of small centers such as Empalizada and Apaneca in the southern sector of the field occurred during the early Brunhes (0.77 &-0.07 Ma). Basaltic-andesitic activity associated with the Cuyanausul volcano took place earlier, i.e. during the Matuyama chron, possibly around 1.3 fO.15 and 1.7 +0.3 Ma.The local igneous basement is composed of Late Miocen*Pliocene andesites, ignimbrites and volcano-sedimentary deposits. Normal polarities and a K-Ar date of 7.37 + 0.73 Ma indicate that the volcanic activity in the study area extends beyond the Gauss chron. The polarity of some of the units in the post-caldera sequence and in the Concepcibn de Ataco and Cuyanausul sequences suggests that they may have recorded short polarity subchrons. 0 1997 CNR.