P. rubens is a common fungus of indoor environment. Along with Cladosporium halotolerans and Aspergillus niger, it is one of the nuisance moulds when humidity is high. It is the most resilient mould as it needs less water for growth and propagation. It has a soft and velvety surface. The spore-bearing filaments, conidiophores are smooth and measure 200-300 µm in length. The hairy surface, penicilli are 8-12 µm long. The conidia are smooth-walled, ellipsoidal in shape, measuring 2.5-4.0 µm long, and are blue or bluish-green in colour. It exists in a number of strains, of which the most important are Fleming's strain  (designated CBS 205.57 or NRRL 824 or IBT 30142) from which the first penicillin was discovered and the Wisconsin strain (NRRL1951) obtained from a cantaloupe in Peoria, Illinois, in 1944 and has been used for industrial production of penicillin G. The original Wisconsin strain itself has been produced in a variety of strains.