Abstract Sampling Bracklesham Group sediments of the WhitecliffBay (Isle of Wight) succession has led to the discovery of an assemblage of marine diatoms in a series of clays previously thought to be barren of microfossils. Although preserved as pyritized steinkerns, there is enough detail present to enable identification to generic and even species level in nearly all cases. The flora includes the stratigraphically restricted species Brightwellia hyperborea Grunow, Aulacodis-cus singiliewskyanus Barker & Meakin and Aulacodiscus subexcavatus Hustedt, which allow the assemblage to be placed within the Triceratium kanayae Diatom Zone of the Middle Eocene. This is equivalent to Calcareous Nannoplankton Zone NP15 and confirms a Lutetian age. The assemblage is dominated by the large centric species Fenestrella antiqua (Grunow) Swatman, previously unknown from strata later than earliest Eocene age in NW Europe, and this occurrence thus extends the range of this species. The diatoms include both low-latitude planktic species and more cosmopolitan coastal and nearshore taxa, attesting to strong connections to both southerly, warmer waters as well as to the north and east. Palaeoecological evidence from the diatom assemblage suggests the presence of water stratification, with eutrophic, nutrient-enriched surface water conditions encouraging the blooming of centric diatoms. This event may coincide with a worldwide increase in biogenic silica, coincident with a pulse of cooling and sea-level lowstand, all of which are related to the initiation of thermohaline circulation, as Antarctica split away from Australia and began to cool down. Mass sedimentation of the diatoms occurred at the end of this period, as improved water circulation led to mixing of the water mass and the return of larger calcareous benthic foraminifera. The presence of relatively nearshore, fairly shallow conditions during deposition of the diatomac-eous interval is indicated by the presence of several benthic diatom genera such as Aulacodiscus and Triceratium .