The objectives of this study were the specific taxonomic confirmation of the main olive seed wasp (OSW) attacking commercial olives in the Western Cape, to investigate monitoring methods and seasonal occurrence of OSW, to determine the potential economic damage of infestations, and to ascertain the geographic distribution of OSW in the regions where olives are cultivated in the Western Cape. Morphological and molecular methods were used to identify all the species obtained from cultivated olives at two trial sites near Stellenbosch and Agter-Paarl. Eupelmus spermophilus Silvestri (Hymenoptera: Chalcidoidea) was by far the most frequent and widespread olive seed wasp. Monitoring with yellow sticky traps over three seasons indicated OSW presence in orchards when olives were small enough for OSW to oviposit in, but traps were not reliable indicators of OSW infestation levels. Prematurely dropped olives, collected from February until harvest over three seasons, and olives picked at harvest were examined for OSW infestation. Total yield for each data tree was recorded at harvest, and yield loss due to OSW infestation calculated. Economically significant yield losses occurred sporadically. Fruits collected from both cultivated and wild olive trees indicated that OSW occurs in most olive-growing regions of the Western Cape. OSW does not pose a significant threat to the local olive industry as a whole and cannot be considered a key pest of olives, but during some seasons it can affect production yields where wild olive trees are located near commercial olive groves.