This study aims to the preparation of lightweight concrete with lignocelulosic fibres from Posidonia Oceanica balls. PO balls can be found in large amounts along the Mediterranean shores. Based on previous scientific research, it has been found that they possess have very interesting properties such fire resistance, moisture resistance and thermal resistance and can serve as a raw material for the production of composites at a very low cost. In the current research it is attempted to examine flexural and compressive resistance of the PO fibres concrete. Results obtained from the tests indicate that PO fibres can be used as a substitute of aggregate producing lightweight concrete.