Recent reviews have shown that hybridisation among ant species is likely to be more common than previously appreci-ated, but that documented cases of introgression remain rare. After molecular phylogenetic work had shown that Euro-pean Lasius niger (LINNAEUS, 1758) and L. psammophilus SEIFERT, 1992 (formerly L. alienus (FOERSTER, 1850)) are unlikely to be very closely related, we decided to analyse an old data set confirming the conclusion by PEARSON (1983) that these two ants can indeed form viable hybrids. We show that signatures of introgression can be detected in a Danish site and that interspecific gene-flow is asymmetrical (only from L. niger into L. psammophilus) as inferred previously by Pearson for the southern England site that he studied and from which we also collected data. We compare the ob-served patterns of hybridisation and introgression in the Danish and British site and infer that overlap in nuptial flights in Denmark may have contributed to the higher frequency of introgressed genes relative to the southern England site where nuptial flights are clearly separated in time. We also report the first mating system data for L. psammophilus, showing that this species has facultative multiple mating of queens similar to L. niger. We suggest that L. psammophilus-niger introgression may be much more common than previously appreciated, which would explain that European myr-mecologists have often found it difficult to distinguish between these species at sites where they occur sympatrically. This would imply that multiple accessible field sites are available to study the molecular details of hybridisation and in-trogression between two ant species that have variable degrees of sympatry throughout their distributional ranges