By use of auxotrophic mutants of Penicillium lanosum were prepared the heterokaryonts. From more rapidly growing sectors from heterokaryotic mycelium on minimal medium were isolated 17 diploid strains. They were prototrophic and on the whole they did not differ from the wild strain in their morphology and their growth rate. All diploids had considerably larger spores than the wild strain. Ovicidity of one half of the diploids, as well as ovicidity of the wild strain was inconspicuous and low. Ovicidity of the second half of diploids was either medium or higher, the 2nd and 3rd type. The set of these diploid mutants will serve for another study of genetic determination of ovicidity.