Summary The ultrastructure of Trimastix marina is documented for the first time. This protist has four flagella which insert at the head of a longitudinal feeding groove. The basal bodies are elongate and arranged in a cruciate pattern. The anterior and posterior basal bodies lack cartwheels. The posterior flagellum bears two vanes. The anterior flagellum is thickened. The cell has a poorly organised Golgi-like region and small hydrogenosome-like organelles. The flagellar apparatus includes left and right major microtubular roots which support the margins of the groove, and a third, major anterior root associated with a fan of microtubules which supports the dorsal side of the cell. There is also a singlet ‘root’ associated with the groove, a minor anterior root and a suite of non-microtubular structures (including the A, B, C and I fibres). Arrays of microtubules diverge from both left and right roots to support the floor of the groove. Some microtubules from the right array converge to form an inner right root. Halfway down the cell the left root ends, while the right root reduces and associates with a sheet-like striated/dense composite fibre. The groove ends as a cytopharynx, with a ‘tongue’ structure associated with its opening. Trimastix marina is most similar to T. pyriformis, the only previously studied member of Trimastix. Among the other ‘excavate taxa’, Trimastix is most similar to ‘core jakobids’, Malawimonas, Carpediemonas and retortamonads but also shares several unusual features with some heteroloboseids. The complexities and detail of structural similarities amongst various excavate taxa support the ‘excavate hypothesis’ (i.e. that the excavate taxa have a common excavate ancestor) but suggest that understanding the relationships amongst the excavate taxa through morphological data will be a complex task.