The invention relates to a biologically-degradable glass fiber composition having good mechanical properties, good workability and in particular fiber-forming-capabilty features and resistance to humidity. The concentrations expressed in percent by weight for each component being the object of the invention are: SiO2: 61 to 66; Al2O3: 1.1 to 2.1; (CaO+MgO): higher than 9; (Na2O+K2O): higher than 18; B2O3: 4 to 15; P2O5: 0 to 5; SO3: 0 to 1; Fe2O3: 0 to 0.5; Others: less than 2.
