A dinocyst assemblage from the Warlingham borehole, Surrey is described. One new species, Australisphaera fragilis is erected. The genus Australisphaera and the species Taleisphaera hydra and Subtilisphaera terrula are emended. In contrast with other known dinocyst assemblages from the 'non-marine' Wealden, the majority of the species discovered are known from truly marine sediments. The varying morphologies and fluctuating diversity of the dinocysts in the study samples allow a number of palaeoenvironmental conclusions to be drawn. These conclusions conflict with the observed distribution of low-salinity ostracods, which are now believed to be hydrodynamically controlled in their distribution. Evidence of a Boreal source for marine incursions into the Wealden basin during Weald Clay times is presented. The dinocyst assemblage is interpreted as being late Hauterivian in age.