Although they are built from similar structural components, the actomyosin-based contractile filaments of smooth and striated muscle are organised very differently: striated muscle contains densely packed, parallel myofibrils, whereas smooth muscle has a lattice-like network of filaments. It is not known what regulates myofibril vs lattice formation, but Shoichiro Ono and colleagues (p. 2662) now show that, in C. elegans, two splice variants of the actin-filament-severing protein UNC-60 (cofilin in mammals) are important in this process. The authors examine smooth muscle (the myoepithelial sheath of the somatic gonad) and show that actin filaments are severely disorganised when the UNC-60A isoform (which has weak actin-severing activity) is knocked down. This phenotype is not rescued by wild-type UNC-60B (which has strong actin-severing activity); notably, however, a mutant UNC-60B that is deficient in actin severing – and therefore has UNC-60A-like activity – rescues the phenotype. By contrast, in striated muscle (the body-wall muscle), wild-type UNC-60B is required for actin-filament organisation. Thus, smooth and striated muscle appear to require different levels of actin severing by UNC-60; this might, the authors propose, be related to the distinct architecture of the contractile apparatus in the two cell types.