AbstractSome fundamental observations and studies have been made on the fruiting ofCollybia velutipescultivated in a chemically defined synthetic liquid medium. Attention was paid to the influences of the concentration of carbon and nitrogen in the original culture medium, the contents of both types of nucleic acids in the mycelial cells on fructification. Besides, the effects of antibiotics, growth hormones, light, and antagonism of other microorganisms were also studied.The cultivating time seemed to be the most important factor which affects the influence of the original nutrient level of the medium on both the mycelial growth and the fruit body production. Thus, in general, more mycelia grow on a medium with higher nutrient level after longer cultivation, and more fruit bodies are also produced on the same medium after the mycelial growth ceased.The nitrogen in fruit bodies seemed to originate in those of medium and mycelia, and N‐fixation seemed to be not obliged to take part in the growth of fruit body. The CO2‐fixation, however, was found to be essential for the normal growth of fruit body.The contents of ribonucleic acid (RNA) and protein in the mycelial cells in a flask reached their maximum value when the first fruit body was just produced, while the deoxyribonucleic acid (DNA) still continued to increase for a period thereafter, and the protein‐N released from the mycelium was supposed to be used for the protein synthesis in the fruit body. The ratios of RNA‐P/DNA‐P and protein‐N/DNA‐P in the mycelial cells had begun to decrease before the fruit body was going to grow. The fruiting was, therefore, expected to be preceded by the deprivation of the ability, of protein synthesis of the mycelial cells.The visible light of shorter wave lengths (i.e., violet and blue light) was found to be indispensable for the normal development of pilius and the production of spore. It was found that this effect of light could by no means modified by supplying amino acids, protein hydrolyzate, or extract of normal fruit body to the medium or on the pilius. Similar clustered secondary fruit bodies were formed by adding tryprophane or indolacetic acid to the medium or smearing them on the minute pilei of young fruit bodies. This abnormal development of fruit body was regarded as a part of the nutritive growth of the stem of the normal one due to the existence of nutritive growth hormones, and the photoinduction was supposed to result in the destruction of nutritive growth hormones or the formation of some growth hormone antagonists rather than the formation of transmissible floral stimulants, such as florigen.The application of some antibiotic substances (e.g., aureomycin and terramycin) to the mycelium on a malt agar culture was found to promote the fruiting, while penicillin showed no effect. The antagonistic actions of a species ofpenicilliumandaspergillus oryzaewere also found to promote the fruiting at their boundary on a malt