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SUMMARY: Four non-penicillin-producing (p) strains were isolated from the strain 47.1564 Wis. of Penicillium chrysogenum and used in this work. Seven heterokaryons and seven heterozygous diploids were synthesized between the four p strains and one penicillin-producing (+) strain. All these strains were further marked by nutritional requirements and differences in colour of the conidia. None of the heterokaryons and diploids between non-producing strains produced any appreciable amount of penicillin; all the heterokaryons and diploids between a producing and a non-producing strain did produce penicillin in amounts of the same order as those of the producing strain. The four p mutations behave like alleles: p is recessive to the wild condition. Linkage of the allele p to y and to cy may be deduced from data of the somatic segregation from a heterozygous diploid.