The dinoflagellate family Warnowiaceae has often been considered to include some of the most complex cells among the protists. The number of described species is around 40, but both the species and generic concepts are in need of revision. Warnowiaceans are often fragile and readily change morphology under the light microscope, and they are usually regarded to be rare. They are particularly famous for the eye-like structures, termed ocelloids. Studies on warnowiaceans are hampered by lack of cultures, and our studies are therefore based on cells obtained directly from field samples. We provide a description of Nematodinium parvum comb. nov. (syn. Pouchetia parva, Warnowia parva), based on light, scanning and transmission electron microscopy, combined with phylogenetic analyses of ribosomal genes. It was described in 1908 by Lohmann from Kieler Bay, but is often common in Danish waters, allowing observations on distribution and behaviour. Crucial conditions for finding high cell abundances were periods of warm temperatures and a calm sea. Cells were yellowish, photosynthetic, and contained a net-like chloroplast, in addition to an ocelloid, trichocysts and harpoon-like nematocysts. They divided asexually but planozygotes were also seen. Following the demonstration of nematocysts, the species, which has been known as Warnowia parva since 1928, is transferred to Nematodinium. The finding of a peduncle indicates mixotrophy but all feeding experiments failed to identify a suitable prey. The phylogenetic analyses based on single-cell PCR and sequence determinations of small and large subunit rDNA confirmed that the systematics of Nematodinium and Warnowia is in a state of flux.