Glucose oxidase (GOD) production by 84 strains of the genus Penicillium was studied in order to define the distribution of the activity among and within the species. Plate screening and sub‐screening in shake culture showed a rather homogeneous behaviour of strains belonging to the same species. Penicillium chrysogenum, P. expansum, P. italicum and P. variabile had a concomitant good production of GOD and catalase activity; peroxidase and polyphenol oxidase were never present. The capability of the culture filtrates to oxidase substrates other than glucose was also studied.

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Conidia of Penicillium variabile P16 were subjected to mutagenesis and selection for glucose oxidase production on media containing o‐dianisidine. Studies of the relationship between dose of UV irradiation and conidial survival and frequency of mutation showed that the best frequency of positive mutation (17%) was obtained in correspondence to a conidial survival of 52%. Out of 54 overproducing mutants tested in shaken flasks, M‐80.10 showed the highest level of glucose oxidase activity (127% higher than the wild‐type). M‐80.10 mutant, transferred every 15 days to fresh medium and tested monthly for 8 months, appeared stable. The time course of growth and enzyme production by the mutant M‐80.10 showed an increase of the glucose oxidase activity in the culture medium up to 19 U ml−1 after 96 h of fermentation.