The ant species Lasius (Lasius) brunneus (Latr.) and L. (Dendrolasius) fuliginosus (Latr.) are known mutualists of the myrmecophilous giant tree aphids of the genus Stomaphis Walk. Species of the Lasius subgenus Chthonolasius Ruzs., which live cryptically underground, have not been considered regular mutualists of those aphids. They often nest close to the known mutualists with which they are through their mutual colony developments intimately interrelated – as temporary social parasites of L. brunneus or temporary hosts of L. fuliginosus. Such a constellation raises the question about contacts of L. (Chthonolasius) species with Stomaphis aphids. Here, we describe modes of interaction between L. (Chthonolasius) umbratus (Nyl.) and S. graffii Cholod. which is usually hosted by L. (L.) brunneus. Our field data from Poland and elsewhere in Europe, together with published information, support our view that also L. (Ch.) umbratus and other underground-living species of the subgenus Chthonolasius are true mutualists of Stomaphis which take care of the aphids. Such ants differ by their protective behaviour from ants which prey on or only exploit aphids. The co-occurrence of L. (Chthonolasius) species with Stomaphis aphids has hitherto been relatively rarely reported, which probably is due to the cryptic lifestyle of these ants. We discuss the complex interrelations among the above Lasius F. species which belong to separate levels of social parasitism, and their relations with Stomaphis aphids. We also suggest that L. (Chthonolasius) species, which in the socially-parasitic hierarchy are located between Lasius s. str. and L. (Dendrolasius), may serve as transmitters of Stomaphis aphids from the former to the latter species.