Abstract: The two monogynous (single-queened), parapatric ant species Temnothorax nylanderi (FÖRSTER, 1850) and T. crassispinus (Karavajev, 1926) are both characterized by a rather inefficient, environment-based nestmate recognition system, which does not prevent alien colonies from moving in with an unrelated colony when their own nest has decayed. Colony fusion results in a genetically heterogeneous colony, in which later one of the two queens is eliminated. The sporadic occurrence of mixed colonies with workers from both species or from a parental species and a presumed hybrid colony suggested that interspecific fusion may occasionally occur in the narrow contact zone in the Franconian Jura. Colony fusions could be observed after initial aggressions in one third of our laboratory experiments. One of the two queens was usually killed within a few days. Queens of the two species did not differ in their survival rate. Apparently, the close relatedness between the two species and the environment-based recognition system facilitate such heterospecific fusion.