In 2015, in the territory of Kyiv (Ukraine) and in 2012-2017 in Transnistria (Moldova), two mixed colonies consisting of two species of ants – Lasius fuliginosus and L. umbratus were observed. Another 2 mixed colonies were discovered in 2021 in the territory of the city of Rivne (Ukraine). The aim of the study was to describe the observed mixed colonies, analyze the interaction of workers of two species on the trails, and by mathematical modelling to determine the probability of preserving queens of both species as part of a mixed colony. On the trails of the colonies from Kyiv and Rivne, the number of workers of L. fuliginosus is slightly higher than that of L. umbratus, so is the number of L. fuliginosus workers in the colony. Workers of both species were making contact both within the same species and interspecific contacts, but trophobiosis with aphids has been recorded only by L. fuliginosus. The most likely of the 4 options considered was the survival of the queens of both species, otherwise the dynamics of the population over 6 years of development (total population of the mixed colony from Kyiv: 27 thousand workers of L. umbratus, 72 thousand workers - L. fuliginosus and two colonies with same population from Rivne) is difficult to explain. The possibility of such mixed colonies co-existing for at least a few years is debated.