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Although it is well known that polynoid scaleworms can autotomise various appendages and posterior portions of the trunk, this is a neglected component of their behavioural repertoire. Autotomy in polynoids and other annelids necessitates the fracture of the external cuticle, which is a collagenous structural material. The aim of this investigation was to gain insight into the mechanism by which the cuticle is fractured during trunk autotomy in Harmothoe impar using light and electron microscopy. At autotomy the cuticle fractures at a pre-formed breakage zone which passes through a cuticular thickening consisting of an internal apodeme and external ridge. It is hypothesised that cuticular fracture is initiated by the contraction of specific groups of longitudinal muscle fibres, which results in the concentration of tensile stress at the basal edge of the breakage zone. The risk of accidental fracture of the breakage zone is reduced by several morphological features including the external ridge, which prevents stress concentration at the external edge of the breakage zone, and the undulating profile of the breakage zone, which increases its resistance to shearing forces. How to cite this article: Wilkie I.C. 2020. Fracture promotion versus prevention: the cuticular breakage zone at the trunk autotomy plane of the scaleworm Harmothoe impar (Annelida: Polynoidae) // Invert. Zool. Vol.17. No.1. P.76–92. doi: 10.15298/invertzool.17.1.08