The conidium of the entomopathogenic fungus, Nomuraea rileyi, has been found to germinate rapidly in the presence of a host insect-derived extract. This extract therefore appears to contain an important factor involved in host recognition by N. rileyi, although the substance (germination-accelerating factor, GAF) responsible for such unique germination behavior has yet to be identified. Our previous study was extended to the isolation of GAF from pupae of the silkworm, a host insect of N. rileyi. This present work subjects GAF to a structural analysis. The chemical structure of GAF is characterized as 2S-amino-tetradeca-4-ene-1,3R-diol (D-erythro-C(14)-sphingosine) based on spectroscopic data. An examination of the structure-activity relationship shows that the activity of D-erythro-C(14)-sphingosine was superior to that of sphingosines with shorter and longer carbon chains. It is suggested that the molecular species with a 14-carbon chain of a sphingosine is important for host recognition.