Pioneering studies in north Texas and adjacent Oklahoma played an important role in the paleontologic and stratigraphic development of the Cretaceous in North America. We have used the results from 135 strontium isotope analyses from fossils and rocks collected from 43 sites to define the “middle” Cretaceous path of seawater 87Sr/86Sr during Comanchean time. The section represents an estimated duration of 19.3 m.y., from late Aptian through late Cenomanian. The 87Sr/86Sr of seawater rises from 0.707318 ± 13 (errors given × 10–6) in the basal Glen Rose Formation to reach a peak value of 0.707522 ± 10 in the Walnut Clay, then gradually declines to 0.707468 ± 6 in the middle Grayson Marl. The upper Grayson Marl and lower Buda Limestone 87Sr/86Sr values decline to 0.707422 ± 9, then rise to near 0.707473 by the middle Buda Limestone. The 87Sr/86Sr mean of the basal Woodbine Formation, in the overlying Gulfian Series, is in agreement with the middle Buda value. Thirty of the analyzed samples yielded ratios higher than the mean determined for corresponding stratigraphic units. The 87Sr/86Sr ratios from whole-rock limestones and spar fillings of shells are inconsistent and more radiogenic than the best shell material. All but two samples yielding aberrant ratios could be identified by using appearance and trace element content. Isotopic results from oyster and pectin shells with low Mn and Fe concentrations are remarkably consistent. The agreement of results among samples collected from a stratigraphic interval at a single site with results from geographically removed sites in the same interval is strong evidence that these samples retain the original open-marine 87Sr/86Sr of Comanchean seawater. The seawater variation defined here shows fine structure not apparent in previous work. This definition can be used to better understand the complex relationship and timing between seawater 87Sr/86Sr variation, ocean anoxic events, mid-ocean-ridge production, and subsequent climate and ocean-productivity changes.