A single, low-density (1.58 g/cm3), phosphatic coprolite recovered from a fluvial Triceratops site in the Upper Cretaceous Hell Creek Formation of eastern Montana contains small quantities of minute bone or tooth fragments, kerogenized plant residues (pollen, spores, sporangia, and cuticle), hyphae of probable fungal origin, and small detrital mineral grains in a fine-grained, highly porous matrix. Roughly 30% of the matrix, composed almost entirely of microcrystalline francolite (carbonate-fluorapatite), is composed of thin-walled vesicles of roughly spherical shape, 0.5–3 µm in diameter. These vesicles are interpreted as mineral pseudomorphs of organic particles, probably including fecal bacteria, existing in the original scat. This structurally well-preserved coprolite is likely derived from the scat of a bone-digesting carnivorous animal, contains much or all of the autochthonous apatite of the original scat, and lacks permineralization that commonly produces a densely lithified object of low porosity. This is the first detailed description of a coprolite of this type from Mesozoic fluvial deposits. This evidence supports the view that dietary calcium phosphate could precipitate rapidly in the scat of ancient carnivorous animals, providing the structural strength to allow preservation of internal organic forms in great detail.