The burr-seed fly Euaresta aequalis (Loew, 1862) is a fruit fly native to North America and has recently been found in the Palaearctic region in western Slovenia and north-eastern Italy. The larvae of this species live in burrs of Xanthium strumarium Linnaeus s. l., destroying its seeds. For that reason, E. aequalis is considered as a beneficial species and is sometimes used for biocontrol of its host plant. In July 2008, E. aequalis was collected for the first time in western Slovenia near Nova Gorica on X. strumarium subspec. italicum (Moretti) D. Löve. Several additional findings in this part of Slovenia and in north-eastern Italy in 2012 show that E. aequalis is already widely distributed and has become established as an alien species in the region, although these records have not been published to date. The records of E. aequalis reported here are the first in the Palaearctic region, and Xanthium strumarium subspec. italicum is believed to be a new host plant of this fruit fly. The infestation rate and the life history of E. aequalis have been studied and are discussed in the context of data available from the literature. The egg, third instar larva and imago are described, and the life history is briefly discussed.