The invention broadly relates to the use of .alpha., .beta.-unsaturated fatty acids to inhibit the filamentous growth of fungi and yeasts and to a method for producing same. In particular the invention relates to the use of optionally substituted C8 to C15 .alpha., .beta.-unsaturated fatty acids or salts, esters or amides thereof for inhibiting or retarding the yeast-to-mycelium transition of organisms having a dimorphic life cycle.