Several heterozygous diploids were made between genetically labelled derivatives of two strains ofPenicillium chrysogenum which produced relatively large amounts of penicillin and were of divergent lineage. The derivatives were labelled with spore colour and nutritional mutations. The diploids, although uniform in having wild type spore colour and being prototrophic, ranged from types having penicillin yields close to that of the original parents to types having less than a quarter of this titre level. Intermediate types had titre levels of about half to threequarters that of the high yielding diploids. Segregants were selected which had arisen naturally and also after nitrogen mustard treatment; most had the spore colour and auxotrophic phenotype of one or other immediate parent. From diploids of low and intermediate titre only haploid segregants with the genetical markers of one parent could be recovered with intact penicillin yield; haploids with the genetic markers of the other showed a marked reduction in yield. However, from diploids of high yield, both parental types could be recovered showing no loss of their original penicillin yield. The bearing of these results is discussed on the suggestion that different degrees of homozygosity between diploids may account for the titre variation observed. An alternative suggestion that mutations suppressive to penicillin titre might cause such variation is also considered.