Entomopathogenic fungi are an interesting object of research due to their great potential for biological control of plant pests and medically important arthropods. As a group, they play an important role in arthropod population density regulation processes together with predators, parasites and other microbial pathogens. A number of species have been selected to be applied in biopesticide technological production. The majority of species known in Poland represent the order of the entomophthorales within Zygomycotina and the hyphomycetous anamorphs of Ascomycotina. Apart from their taxonomical diversity they show strong functional differentiation resulting from enzymatic and toxic activity against their hosts and immunological response of the latter. These phenomena may be considerably modified by environmental conditions. Some aspects of these relationships were discussed and greater attention to their significance for pharmacological and other biotechnologies is suggested.