Hyssop (Hyssopus officinalis L.) is an ancient medicinal and aromatic plant that is used mainly for flavouring and food preservation and for phytotherapeutic use. In order to study the effects of different irrigation regimes on growth, biological yield and essential oil yield of hyssop, an experiment was conducted based on randomized complete block design with four replications at the Agricultural Research Station, Ferdowsi University of Mashhad, Iran, during two growing seasons of 2008-2009 and 2009-2010. Three irrigation levels were every 7, 14 and 21 days. Plant height, biological yield, essential oil content and essential oil yield of hyssop were measured accordingly. The results showed that the effects of irrigation levels was significant on (p≤0.01) on plant height, biological yield, essential oil content and essential oil yield.The highest and the lowest biomass yield were observed at 7 and 21 days, respectively. The maximum essential oil content was obtained at 21 days and the minimum was for 7 days.

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