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S ummary Light and temperature greatly influenced the development of vesicular‐arbuscular mycorrhiza and growth of onions in a phosphate‐deficient soil. There were more large arbuscules and host growth was stimulated more with 25,000 lux than with 13,000 lux at 23° C and in a 14‐23° C diurnal cycle. At 14° C and 13,000 lux mycorrhiza caused no growth stimulation even in three low‐phosphate soils. At 18° C infection was much sparser in a 6 h daylength than in 12 and 18 h. Mycorrhizal Plants kept in daylengths of 6 h at 13,000 lux were 3.5 times heavier than their non‐inoculated controls. The effect of infection increased in longer daylengths and higher light intensities to 14.2 times the weight of controls with 18 h at 25,000 lux. The addition of soluble phosphate stimulated growth to the same extent as mycorrhizal inoculation did in the highest light conditions but phosphate stimulated growth more than did mycorrhiza under intermediate light conditions. The amounts of soluble carbohydrate in the roots of Plants given phosphate and in those that were mycorrhizal did not differ significantly, but there was more soluble carbohydrate in Plants growing in most light. Plants both with and without mycorrhiza contained much glucose, fructose, sucrose and an unidentified sugar with a low R F value, but there was no indication of fungal carbohydrates such as trehalose and mannitol.