A palynological investigation of the Lower and lowermost Middle Jurassic (Sinemurian to Aalenian) strata of North Yorkshire has been undertaken which incorporates material from the BGS Brown Moor Borehole, near Acklam and the coastal outcrops between Staithes and Robin Hood’s Bay. The resulting dataset is an additional contribution to the Jurassic dinoflagellate cyst biostratigraphy of northern England. Two distinctive variants of Liasidium variabile appear to have biostratigraphical significance in the Upper Sinemurian of North Yorkshire. Specimens of L. variabile from the Caenisites turneri Ammonite Zone, the Eparietites denotatus Ammonite Subzone and the Oxynoticeras simpsoni Ammonite Subzone have short apical and antapical horns. Forms from the overlying uppermost Oxynoticeras oxynotum Ammonite Zone, however, are characterized by elongate apical horns and rounded hypocysts. The regional dinoflagellate cyst diversity increase in the late Pliensbachian and the disappearance of many dinoflagellate cysts in the Lower Toarcian are both interpreted as being related to eustatic fluctuations. The paucity of dinoflagellate cysts close to the Dactylioceras tenuicostatum-Harpoceras falciferum Ammonite zonal transition in the Lower Toarcian was caused by the spread of widespread marine anoxia during the Lower Toarcian maximum flooding event. A new species of dinoflagellate cyst, Nannoceratopsis symmetrica, is described.