Our results provide a shift in our understanding of how archaella assemble and function. We report the structure of a heteropolymeric archaeal filament composed of two alternating subunits, ArlB1 and ArlB2 and provide insights into the dynamics of archaella. Here we employed electron cryo-microscopy (cryoEM) to solve the structure of the archaellum filament from Methanocaldococcus villosus, a hyperthermophilic archaeon 11 and one of the fastest swimming organisms known 37. However, not all archaea show this feature, including those that encode only one archaellin. salinarum 36, minor archaellins form a region at the base of the archaellum filament that is reminiscent of the flagellar hook. Genetics and molecular biology experiments suggest that in M. The roles of minor archaellins mostly remain enigmatic. Nevertheless, the three high-resolution structures of archaella that are available to date show homopolymeric filaments consisting of only a single (major) archaellin 20, 33, 34. Biochemical and genetic experiments suggest that in Methanococcus voltae 28, Methanococcus maripaludis 29, Methanothermococcus thermolithotrophicus 30, Halobacterium salinarum 31 and Halorubrum lacusprofundi DL18 32 the archaellum filament consists of two major archaellins, hypothesised to be equally distributed along the filament. Archaellins that are thought to form the bulk of the filament are referred to as major. Conventionally, ArlA and ArlB archaellins are divided into major and minor structural components of the filament. When multiple archaellins are encoded, several or all can be transcribed at the same time 26, 27. The number of archaellin genes is species-specific, usually varying from one to seven 24, 25. The first genes in this operon ( arlA or arlB) encode for the archaellin subunits that make up the archaellum filament. Most genes encoding the archaellum machinery components are organised in one arl operon. In the periplasm, the motor complex is anchored to the S-layer by the ArlF and ArlG stator proteins 22, 23. The motor is surrounded by a cytosolic ring of ArlX proteins (in Crenarchaeota) 19 or ArlC, D/E (in Euryarchaeota) 20, 21. Filament assembly and rotation is an ATP-dependent process driven by the motor complex, consisting of the platform protein ArlJ, the hexameric AAA + ATPase ArlI, and a putative regulator ArlH 6, 16, 17, 18. The filament is a helical array of ArlA and/or ArlB archaellins 14 that assemble proximal to the cell surface 15. The archaellum machinery consists of a membrane-embedded motor complex and a ~10 μm long filament, which together are formed by ~10 components, depending on the species 13. Other functions of T4F in archaea include surface adhesion 8, 9, 10, 11, cell–cell contacts and biofilm formation 8, 12. Within the T4F superfamily, archaella are unique in providing swimming motion by means of a rotary propeller. The T4F superfamily includes a vast range of filamentous nanomachines, such as bacterial type-IV pili (T4P) 6, associated with diverse functions 7. Instead, archaella are structurally homologous to filaments of the type-IV filament (T4F) superfamily. While archaella, like bacterial flagella, mediate swimming, the two systems are not homologous, suggesting their independent evolution. A clockwise rotation of the archaellum moves the cell forward, whereas a counterclockwise rotation moves the cell backward 5. The archaellum is a propulsive nanomachine consisting of an intracellular motor that drives the rotation of an extracellular filament 4. Archaea are ubiquitous microorganisms that successfully colonise diverse environments partially due to their ability to swim by means of archaella 1, 2, 3.