SUMMARY Sedimentation during the Lower Oxfordian in North Yorkshire proceeded in two contrasting phases. The first, a shallow water sedimentation phase of the scarburgense Subzone age, was succeeded by deeper water conditions as the Callovian/Oxfordian transgression reached its maximum extent and the Oxford Clay facies reached the area. The second phase reversed this situation, with the gradual silting up of the basin, the accumulation of large thicknesses of Rhaxella spiculite (Lower Calcareous Grit), and then a gradual change to oolite sedimentation at various parts of the basin. A major non-sequence beneath the cordatum Subzone sediments results in the costicardia Subzone being absent from large areas. At the close of the Lower Oxfordian, the major part of North Yorkshire was undergoing oolite sedimentation. North Yorkshire comprises the type area of the Oxfordian Stage, and, as such, yields excellent representatives of the ammonite faunas which characterise the five subzones of the Lower Oxfordian. Many of these species are figured here, often for the first time from North Yorkshire.