Observations have been made on the influence of humidity on spore germination, the rate of elongation of the germ tubes and the production of asexual and sexual fructifications of various mould fungi. The humidity range at which the different species developed is given. Some of the Mucorales and certain pathogenic moulds required high humidities (100‐93%) for their successful growth.Penicilliumspp. had a wider humidity range (100‐80%).Aspergillusspp. were able to grow under conditions of even more restricted moisture supply. Spores of members of theA. glaucusgroup, particularly, germinated and developed at humidities as low as 70‐64%. The reasons for variations in the humidity range of the different species are discussed. The influence of the supply of nutrients on spore germination and subsequent mould growth is consi