The structure of ficellomycin, an antibiotic previously discovered by Argoudelis et al., is elucidated as valyl-2-[4-guanidyl-1-azabicyclo[3.1.0]hexan-2-yl]glycine (1) by NMR, MS, and derivatization studies. The 1-azabicyclo[3.1.0]hexane moiety in 1 represents an unusual ring system making ficellomycin a unique natural product compound.