Abstract: Method for the production of mineral fibers resistant to alkaline media which are to be used for the micro-reinforcement of cement-, lime- and/or plaster-bound or bituminous binders or building materials. The starting material is a mixture of extrusive rocks with an addition of 4 to 6 weight percent of ZrO.sub.2, based on the end product, in a composition that is readily fusible and easy to process into fibers of high mechanical strength and resistance to alkaline media. The mixture is fused at a temperature between 1350.degree. and 1500.degree. C. and is then processed into fibers 10 to 20 .mu.m in diameter.