Abstract Chromosome numbers for 12 Hieracium s.str. species from Bulgaria, Macedonia, Montenegro, Poland, Romania and Serbia are given and their metaphase plates are illustrated. Chromosome numbers are published for the first time for H. albinum Fries 2n=4x=36, H. bukovicae Rohlena & Zahn; 2n=3x=27, H. retyezatense subsp. subatratiforme Neičeff & Zahn 2n=4x=36, H. velenovskyi Freyn 2n=3x=27, as well as two undescribed species belonging to the H. bohatschianum agg. 2n=4x=36 and H. nigrescens agg. 2n=4x=36 and a hybrid between H. naegelianum Pančić and H. bifidum s.lat. 2n=4x=36.