The Cossidae are an economically important family of moths consisting of six subfamilies and more than 700 species globally. Their larvae are often of serious concern to forestry and horticulture in that they are known to bore in the branches and trunks of a wide range of trees and shrubs. Cossulinae is one of the six subfamilies of Cossidae and is restricted to the New World. As a result of this revision, the Costa Rican Cossulinae has been found to consist of five genera and 18 species. The phylogenetic relationships of all Cossulinae genera were analysed using the computer programs WinClada and NONA. Glandular organs new to Cossidae and the Lepidoptera have also been discovered. Four genera, Biocellatagen. nov., Magulacra gen. nov., Simplicivalvagen. nov. and Spinulata gen. nov., and the following nine species are described as new: Biocellata bifidasp. nov., Biocellata davisorum sp. nov., Cossula buspina sp. nov., Cossula duplexata sp. nov., Cossula longirostrum sp. nov., Cossula minutiloba sp. nov., Simplicivalva ampliophilobia sp. nov., Spinulata oblongata sp. nov. and Spinulata quasivinnea sp. nov. © 2008 The Linnean Society of London, Zoological Journal of the Linnean Society, 2008, 154, 222–277.