Toxisarcon alba n. sp., is a giant, naked marine foraminiferan recorded at two locations on the west coast of Scotland, UK. Individuals were found buried or semi-buried in well-oxygenated silty sand at 10 to 30 m water depth. A distinctive, slightly raised mound of 18–40 mm diameter, through which there were many perforations, indicated the presence of an individual on the seabed. Partial removal of the overlying sediment revealed the naked opaque, white, sticky cell body. Toxisarcon alba exists in three distinct forms. In the most commonly occurring form (and the only form observed in the field), the central portion of the cytoplasm extends peripherally into numerous large cytoplasmic extensions and reticulopodia (branched form). In aquaria, some individuals transformed into a consolidated form (spheroid form) by retracting most or all of the reticulopods resulting in a cell with a very well defined edge. On four occasions a third very diffuse form, resembling a fungal mycelium, was observed where reticulopodia were spread over the sediment surface. The closest related species, based on analysis of ribosomal DNA sequences, is the newly described Toxisarcon synsuicidica.