ABSTRACT: Across much of New Mexico, the Lower Permian Yeso Group is a thick and complex succession of nonmarine fluvial and eolian, evaporitic coastal sabkha and shallow, restricted to openmarine deposits.Yeso strata have long been assigned a Leonardian age because of their lateral equivalence to Leonardian strata of the Bone Spring Formation in the Delaware Basin of southeastern New Mexico-West Texas. However, Yeso strata have also failed to yield many age-diagnostic fossils.We document the first conodont assemblages from Yeso strata, from the lower part of the Los Vallos Formation of the Yeso Group in the Caballo Mountains of south-central New Mexico. These conodonts represent the Neostreptognathodus clinei-N. prayi Zone of middle Kungurian age. The conodonts from the base of the Los Vallos Formation correlate to a position near the base of the Cathedral Mountain Formation of the Leonardian stratotype and indicate that the LosValles Formation is approximately middle Leonardian in age. This correlation also establishes the age of an extensive marine transgression across much of New Mexico.