DNA sequences homologous to the maize Activator (Ac) element are widespread in plant genomes. Nowadays, several reports are available concerning the distribution and characterisation of Ac-homologous sequences in natural populations of different cereal species. but these mobile genetic elements still remain to be comprehensively characterised. In this respect, there is a particular lack of information about the dynamics of Ac-homologous sequences within mutant germplasm collections. Here, we present data on the genomic diversity and methylation patterns of Ac-homologous sequences in ethyl methanesulphonate (EMS)-induced sphaerococcum mutant forms of common wheat (Triticum aestivum L.) and triticale (X Triticosecale Witt.). The results show that the initial EMS treatment has influenced the wheat genome stability by enhancing the dynamics of Ac transposon-homologous sequences.