Summary:Heat resistant moulds of the generaByssochlamys, Aspergillus, PenicilliumandPaecilomycescan be cultivated selectively in the presence ofBacillusspp. when at least 10 p/m of chloramphenicol or chlortetracycline is added to the growth medium. The inhibitory agents sulphafurazole, penicillin, streptomycin, oxytetracycline and furazolidone, do not give completely selective and optimal growth of these moulds. Selective cultivation of heat resistant moulds from 200 samples of Dutch soil showed thatByssochlamys niveaoccurs more frequently than was expected, whereasByss. fulvaoccurs by way of exception. Other heat resistant moulds observed in soil are, in order of frequency, species of the generaAspergillus, PaecilomycesandPenicillium.