Pristine streams of a Pannonian lowland mountain support high mayfly species richness with diverse assemblage structure, which contributes significantly to our knowledge of mayflies in South Eastern Europe. Moreover, records of symbiotic parasite Symbiocladius rhithrogenae (Zavrel, 1924) attached to nymphs of Rhithrogena group semicolorata, represent new data for species distribution and show that our knowledge of the Croatian chironomid fauna is still growing.