Abstract: The development of the fruit bodies of Gyromitra esculenta (Fr.) Pers. was observed in the field in Central Finland (62° 18′ N, 26° 15′ E). The first fruit bodies become visible when snow and ice are still present in the forest. The initial stages and very small fruit bodies were seen to develop during the previous year. The authors believe that there is a mycochrome in G. esculenta which reacts to the short daylengths in autumn and inhibits the de velopment of the young fruit bodies, but which induces their growth in the spring, as the day is lengthening.