The hitherto undescribed ovipositing behaviour and eggs ofSimulium dentulosumRoubaud are described. The laying of eggs under a thin film of water on the vertical face of a weir wall represents a specialized form of ovipositing which ensures that the larvae are placed in their favoured habitat. It also confirms that this species is entirely adapted to developing in torrenticolous waters. The eggs swell by 51% of their original volume during development and are considered vulnerable to desiccation. The reduced size of adult female mouthparts suggests that the population of this species is autogenous.