The regularities of biosynthesis of 6-aminopenicillanic acid (6-APA), benzylpenicillin (BP) and phenoxymethylpenicillin (PMP) by the strains under the investigation did not significantly differ. In the absence of the precursor both the strains mainly synthesized 6-APA. Phenylacetic acid (PAA) and phenoxyacetic acid (POAA) provided directed biosynthesis: the fungus synthesized BP or PMP depending on the precursor nature. When the amount of the precursors was not sufficient, 6-APA was synthesized along with the penicillins. PAA proved to be a more active precursor than POAA. When both precursors were present in the fermentation broth, only BR was synthesized. An important distinction of strain 316A was its increased sensitivity to PAA especially in the initial period. After an increase in the PAA concentration the growth rate of strain 316A lowered to a greater extent than that of strain 284A. This was likely to determine the higher levels of penicillin production by strain 316A in the presence of POAA, a nontoxic precursor. A procedure for supplying the precursors was developed. Under the laboratory conditions it provided high levels of the penicillin production.