Alterations in the competitive force of certain basidiomycetous ground fungi as exposed to daylight c;. CRAMSS (Eingeganqen a m 23. 2.1984) Mycelia of 20 basidiomycetous ground fungi were transferred to nonsterile, pasteurized, and hestst,crilized soil and manurc substrates of different fertilit,y. Thc cornpitratjive development of tho mycelia in the dark, in moderate daylight, and in t#lie presence of allelopathic volatiles was dcterm i n d by H semiquantitative method. I n sterile cultures, the dry matter production of the majority of fungal species was uninfluenced by light irradiation, a,lthough in 25% of the test mycelia, tJlie productivity was lowered down to 35% or enhanced to 135% t'o t'lie dark control. Tn nonsterile and pasteurized soil substrates, light irradiation caused dramatic changes in fungal development. Certain groupings of fungi were completely arrested in growth, showcd a drastic reduction of the mycelial density but remained capable of permeating the substrate, reduced the dry matter production moderately down to 21 to 70% to the dark control, or were clearly stimulated in the rangc of 143 to 185%. I n nonsterile optimum substrates such as horse manure and soil of the natural stand from which the fungus had been collectcd, the effect of light inhibition abated in Agaricus crlutis, A . campestris, and Macrolepiota procern. In contrsst, the severity of light inhibition was amplified in the presence of allelopathic chemicnls released from aerobically decaying grass residues and/or the respective decay microorganisms. It is concludcd that in axenic culture on soil, both dry matter production and linear expansion growth of basidiomycetous mycelia are moderately inhibited or stimulated via possible alterations in nutrient uptake, prot'ein and enzyme synthesis, and the activation of the entire metabolism. A light-induccd wcakening of tho fungal physiology and competitive force is further burdened in nonstorilc suhstrat,es by a microflora competing for nutrients and producing toxins inclusivc allclopathic volatiles. The relaxation of light-induced inhibition on optimum substrates may conseqnently be attributable to an improved supply with ndequato nut'rients. reduced competitive stress exerted by the concomitant microflora and the absence of toxic constituents so characteristic of the majority of soil types. The degree of stimulation o r inhibition may further bc influenced by the ligbt repression of the competitive microflora alto wing, on its weakening, the basidiomycet.ous fungus a more successful competition for nutrients in tin environment somewhat, discharged from microbial inhibitor product'ion or cvcn tending to synrrgistic components.