The essential oil content of various populations of MELISSA OFFICINALIS L. (Lamiaceae), cultivated under Mediterranean climatic conditions (Ebro-Delta, Spain) has been investigated during five years of selection and improvement of the genetic plant material. Starting with an essential oil content of 0.2-0.3%, a content of more than 0.5% was obtained as a result of genetic improvement. A weak negative correlation between the content of essential oil and phenotypical growth parameters such as the number of branches per plant and height was observed. No correlation between biomass production per plant and essential oil content could be found. By the method of hybridization, M. OFFICINALIS synthetics with a high yield of biomass and essential oil content were selected.