Background & Aim: Hypericum species are of pronounced ecologic and medicinal importance because it is an essential model system in plant-based medications and, therefore, stimulated the scientific community. The taxonomy of Hypericum is mainly based on morphology and chemical markers. No records have been reported for the Jordanian Hypericum species concerning the chromosome numbers. The present investigation was conducted to assess the cytogenetics of two populations of H. perforatum gathered from different locations in Jordan. Experimental: Seeds of H. perforatum from the National Center for Agricultural Research and Extension seed gene bank were germinated under greenhouse conditions. Root tips of plantlets were squashed, aceto-orcein-stained, and direct microscopic counts of chromosome numbers in meristematic cells were performed. Results: H. perforatum requires specific conditions for germination and survival. Chromosomes are small and tend to clump making accurate counts difficult. No differences were observed between the geographical sites. The individuals from the two investigated populations were predominantly tetraploids (2n=4x=32), some were diploids (2n=2x=16), and a few hexaploids (2n=6x=48). This is the first research on the chromosomes of the Jordanian H. perforatum. Recommended applications/industries: The present finding can be extended and supported by two types of experiments to verify the existence of triploidy in H. perforatum accessions with the number of their mitotic chromosomes. Since the diploid herbs of this species supersede their polyploid counterparts in phytochemical contents, it is important to preserve them with different technologies for industrial purposes. Manuscript profile