Migration and spreading of cells depend on dynamic actin polymerization at their protruding edge, and on actin stress fibres providing contractility through the presence of crosslinkers, such as α-actinins or non-muscle myosins. Different types of actin fibre have been found in cells, including dorsal stress fibres, transverse arcs and ventral stress fibres. Among these, dorsal stress fibres are least understood with regard to their composition or regulation. On page 263, Tea Vallenius and co-workers set out to characterise this stress fibre subtype in human osteosarcoma cells, which have particularly well-defined stress fibres. They find that assembly of dorsal stress fibres specifically requires α-actinin-1, and that its depletion leads to decreased cell migration without affecting the activity of myosin II. This suggests that dorsal stress fibres promote migration in a myosin-II-independent manner, a theory that is further confirmed because dorsal stress fibre trunks lack myosin II and are resistant to myosin II ATPase inhibition. But how are dorsal stress fibres regulated? Rac1 – as a crucial regulator of actin polymerization – is an obvious candidate and, indeed, the authors find that Rac1 induces the assembly of dorsal stress fibres, whereas RhoA induces contractile ventral stress fibres. Taken together, these results provide evidence that α-actinin-1 and Rac1 are crucial mediators in the assembly of dorsal stress fibres in migrating and spreading cells.