A colony of Myrmica rubra (L.) parasitised by the mermithid nematodes was found in Pieniny Mts (S Poland). About 25% of the adult workers were infested, and their body, apart from a visibly distended gaster, was modified to an extent previously unknown in mermithised Myrmica ants (shape of body parts, sculpture, pilosity). The morphology of infested and non-infested individuals was compared using standard measurements and indices employed in taxonomy of the genus Myrmica Latr. Mermithogenic modifications of body structure in Myrmica species are signalled to be the cause of the possible taxonomie problems and mistakes.