In libraries that have never been subjected 10 treatment of the buildings against vermin it is worthwhile to undertake a disinfection with a 1.5 to 2% suspension of DDT. If a library storage area has been treated several times with DDT preparations and the insects have developed resistance to this substance, an organophosphorus preparation, namely chlorophos, should be used as an insecticide. As chlorophos will rather soon lose its toxicity it is more rational to use a mixture consisting of 2-3% chlorophos and a 3% mineral oil emulsion of DDT.