A nesting trace preserved in alluvial floodplain deposits in the Upper Cretaceous Two Medicine Formation at the Willow Creek anticline in north-central Montana contains four crushed theropod eggs referable to the oospecies Continuoolithus canadensis. These eggs immediately overlie the lower surface of a 35-cm-long × 7-cm-thick, dark-green mudstone lens, surrounded by reddish-purple mudstone. The long axes of three eggs are parallel to one another and to the lower boundary of the lens, whereas the fourth egg lies at a 30° angle to the others. A thin, 1-cm-thick organic horizon overlies the eggs, suggesting they were buried with some vegetation. Geometric modeling of the slightly asymmetrical C. canadensis eggs yields a volume and mass of approximately 194 cm³ and 205 g for each egg. This method provides a more accurate estimation for the surface area than allometric equations that are based on modern bird eggs because of the elongate shape of many non-avian theropod eggs. Pore density and water vapor conductance (G_H2O) calculated from one egg in the trace and five additional C. canadensis eggs from the Willow Creek anticline vary across three regions. High, moderate, and very low G_H2O characterize the equatorial zone, blunt, and tapering poles, respectively. The average G_H2O for all eggs exceeds that of an avian egg of similar mass by 3.9×, thus supporting sedimentologic evidence of substrate burial during incubation.