Siluania monomastiga gen. et sp. nov. - one of the smallest free-living heterotrophic eukaryotes (1.5-3.0 μm length) has been investigated using SEM and TEM, and by gene sequence analysis of the small subunit rRNA. The flagellate is covered by only the plasmalemma. The single flagellum has a unilateral array of tubular mastigonemes; the flagellar basal body is only composed of doublets of microtubules. One or two additional very short basal bodies may also be present, composed of single microtubules, of amorphous electron dense material. There is a well developed cytostome with permanent pharynx, supported by rootlet microtubules and a fibrillar sheet. One mitochondrion with tubular cristae contains a spherical body in the centre. Developing mastigonemes are located within the perinuclear space. The Golgi apparatus consists of a single dictyosome. The rootlet system of the flagellum is represented by three microtubular bands, numbered as r1, r3 and r4 in accordance with the protocol accepted for biosoecids, and by the fibrillar rootlet having two strands. The cytoskeleton structure and organelle disposition of S. monomastiga are similar to those of bicosoecids, particularly Cafeteria roenbergensis. Molecular data confirm this view. The presence of a well developed permanent cytostome/cytopharynx complex distinguishes S. monomastiga from other bicosoecids. A new family Siluaniaceae (according to IBCN) and Siluaniidae (according to IZCN) is erected for this species.