Based on study of modern foraminifera, changes in test porosity of ancient planktic foraminiferal specimens may be interpreted to reflect primarily changes in water temperature. To test this hypothesis, planktic foraminiferal porosity data (percent pore space per unit area) of Hedbergella delrioensis from three localities in the Cretaceous Western Interior basin were analyzed in the context of basin history (e.g., relative sea-level). These data record spatial and temporal changes in water masses at the average living depth of H. delrioensis. A significant increase in porosity between the upper Cenomanian M. mosbyense biozone and lower S. gracile biozone suggests an increase in water mass temperature, reflecting northward advection of a central core of warmer (less dense) water into the central seaway. The spread of this water mass toward the north, and laterally east and west, correlates with an independent record of rising sea-level. In addition to this secular porosity trend, higher frequency fluctuations may reflect transient oceanographic events related to changes in sea-level or circulation. In this study, porosity measurement was improved utilizing a scanning electron microscope and image analysis software. The work confirms prior results that as few as five foraminifera are required to produce a statistical assessment of average sample porosity. Use of pore concentration (number of pores per unit area), previously suggested as a temperature proxy, was not supported. The major interpretative contribution is a temporally precise record of spatial changes in Western Interior water masses.