A systematic stable isotopic study of an exceptionally well-preserved Baculites compressus specimen from the Pierre Shale (Upper Campanian) of South Dakota was conducted to assess whether the variations recorded in the calcareous shells of baculitids can be used as a reliable indicator of paleoenvironmental conditions. We sampled the B. compressus conch along four sampling profiles: (1) an ontogenetic traverse parallel to growth direction, (2) a virtually isochronous traverse along two different lirae delineated by growth lines, (3) a septal traverse, and (4) a shell-wall traverse through the conch wall. To date, the ontogenetic sequence is the longest record produced from baculitids. We interpret the isotopic variations determined from the ontogenetic, septal, and depth-profile traverses to be dominated by an environmental signal, especially in shell material deposited prior to septal crowding (i.e., maturity). In contrast, we interpret the isotopic variations determined from the isochronous traverse along the two different lirae to be altered by "biological vital effects." Our results indicate that a careful sampling strategy that avoids strongly ornamented areas of the shell is required if a "true" paleoenvironmental signal is to be extracted from the calcareous shells of baculitids, and perhaps other ammonites as well. Our best estimate of seasonal seawater-temperature variation in the Late Campanian Western Interior Seaway at 42 degrees N, based on the oxygen isotopic data from the baculitid specimen, is 10.0 degrees C (range of 29.7 degrees C to 19.7 degrees C).