SUMMARYThe Scotch thistle, Onopordum acanthium (Asteraceae) has the Eurasian origin and represents an invasive weed in the USA and Australia. It is a serious problem in pastures, along roadsides, rangeland, etc. The weed is very common in Bulgaria and in 2009-2010 several sites with large populations of O. acanthium have been located. The weekly surveys of some of the sites, mainly in the region of Plovdiv, revealed a significant diversity of phytophagous insect species, some of which had very high population density. More than 30 species have already been identified and observations on the phenology and biology of some of them have been conducted. For some of the species, such as endophagous Larinus latus, Lixus cardui, Eublemma amoena, Trichosirocalus briesei, or ectophagous Cassida rubiginosa etc. found during our surveys, there are reports in the literature describing them as having been already introduced as successful biocontrol agents in Australia. Endophagous species like lepidopteran Myelois circumvoluta and Pyroderces argyrogrammos which feed on the stems and the head, and the tephritid flies Tephritis postica and Chaetostomella cylindrica which feed on the capitula, seem quite destructive for the weed, but need to be further studied regarding host specificity. Species like Vanessa cardui and Brachycaudus cardui were found regularly, but in low population density, mainly due to the high rate of parasitism.