Twenty different species of fungi belonging to the genera Aspergillus, Cephalosporium, Dreschlera, Fusarium, Mucor, Neurospora, Rhizoctonia, Rhizopus, Penicillium and Trichoderma were isolated from jute and woven polypropylene sacks using the blotter test, solid medium test and the decimal serial dilution technique. There was a highly significant difference (Students' t-test, P < 0.05) between the larger number of fungal colonies associated with jute sacks than with woven polypropylene sacks. Correspondingly, more fungal species (16) were isolated from jute sacks than from woven polypropylene sacks (9). The blotter test showed that in the absence of an exogenous supply of nutrients, 88% of the sections of jute sacks supported in vivo growth of fungal spores whilst woven polypropylene sacks could not support the growth of contaminating spores. Evidence is presented that mould and yeast counts on sections of new-woven polypropylene sacks incubated at 80 and 90% relative humidity (R.H.) for four months increased by less than 1 log cycle, whilst in similar sections hung on a line under ambient conditions (75 ± 10% R.H. and 28 ± 3°C) viable counts of spores decreased. Sections of fresh jute sacks similarly treated supported 1 log cycle increase in mould and yeast counts at 80% R.H. and 2 log cycles increase at 90% R.H. after four months of storage. Gamma-ray irradiation (4.0 kGy) reduced the mould and yeast counts on new jute and new woven polypropylene sacks by 1 and 2 log cycles, respectively, but post-irradiation storage at 80% R.H. allowed moulds like Aspergillus flavus, A. niger, Fusarium nivale and Penicillium verrucosum var. cyclopium to commence growth on jute sacks. This presumably, may act as a springboard for infecting grain contents of jute sacks. Inert woven polypropylene sacks failed to support fungal growth.