Colour pattern is the most common character to identify species in several large genera of Collembola. Its use often raises problems due to various and poorly investigated extent of chromatic variability among species. Isotomurus festus sp. nov. is here described from Kunashir Isl. (the Kuriles, the Far East of Russia). The species, a member of the 'antennalis' group, is characterized by the lack of trichobothria and slender claws, but is greatly variable in coloration. DNA barcoding (COI) results supports that all the colour forms encountered belong to the same species. While colour pattern has been shown to be the most reliable character for species identification in several Entomobryidae genera, it might not be the case in Iso-tomurus Börner, 1903, the sole large Isotomidae genus where colour pattern is routinely used for taxonomy.