AbstractAnalysis of restriction fragment length polymorphism (RFLP) in mitochondrial DNA (mtDNA) in a population ofLeptothorax acervorumdemonstrates substantial population substructuring. Digestion with four restriction endonucleases, Haelll, Mbol, Mspl and Rsal, gave six, four, three and two different patterns, respectively. Seven composite haplotypes were obtained from the observed cleavage patterns. Uniform aggregations of nests from the same maternal lineage (i. e. with the same haplotype) were found which suggests that nestfounding by a process of budding‐off is common. Groups of nests seem to actively exclude other haplotypes from establishing nests within their territory. Fstatistics suggests that individuals mate at random within the populatio