Important artifacts have been found in situ (i.e., not redeposited) within lacustrine deposits in the Valsequillo region. These deposits contain many diatoms which indicate an age corresponding to the Sangamonian Interglacial sensu lato (80,000 to ca. 220,000yr BP). Two of the four samples in this study are associated with the Dorenberg skull or with stratigraphic units which contain bifacial tools. The remaining two samples are from diatomaceous deposits which are also Sangamonian and stratigraphically above the artifact units. These four diatomaceous samples yielded 30 extinct and 143 extant diatom taxa. The ages of the four samples correspond to other diatomaceous samples (some of which are associated with artifacts) from nearby Valsequillo localities. A post-Sangamonian age for these four diatom-bearing samples is discounted by the presence of Navicula bronislaae and N. dorenbergi, both of which have short stratigraphic ranges and are known only from the Sangamonian (or its equivalents), and by 13 diatoms which evidently have known long stratigraphic ranges and extinctions before the end of the Sangamonian. An age no older than Sangamonian for the artifacts and their enclosing diatomaceous deposits is indicated by the presence of two diatoms (Epithemia zebra var. undulata and Navicula creguti) known only from Sangamonian (or = age) or younger and by an extant diatom, Cymbella cistula var. gibbosa (C. gibbosa), which has its first occurrence in the Sangamonian.