The semaphorins are a family of proteins thought to be involved in axonal guidance. Most of the known semaphorins have a similar primary structure characterized by the semaphorin domain and a carboxy-terminal Ig motif. Here we report the cloning of two members (semF and G) of a novel class of membrane-bound semaphorins which contain seven carboxy-terminal thrombospondin repeats, a motif known to promote neurite outgrowth. SemF and G transcripts are expressed, together with semD and E, in specific regions of young mouse embryos, demarcating distinct compartments of the developing somites or the undifferentiated neuroepithelium. The identification of semF and G increases the number of vertebrate semaphorins to at least 20 and suggests that some semaphorins might act as positive axonal guidance cues.