The ectomycorrhizal fungus Hebeloma cylindrosporum Romagnési produced sporulating fruit bodies under axenic conditions on a synthetic medium when cultivated with its usual host plant Pinus pinaster (Sol.). In the absence of the host plant, H. cylindrosporum only formed primordia or immature fruit bodies. In 11 Erlenmeyer flasks, the fungal fruiting started about one and half months after the inoculation of one-month-old P. pinaster plants by the mycelium of H. cylindrosporum, and a total of 144 fruit bodies appeared in 45 flasks during a 9.5-month fruiting period. At 18 °C, the mean number of basidiomes developed per flask was about 3–5. A thermal shock of ± 6 °C for 5 d did not significantly affect the mean number of basidiomes per flask but modified the time course of their rate of formation. Spores were collected from the fruit bodies and were found to be capable of germinating.