Topological semimetals materialize a new state of quantum matter where
massless fermions protected by a specific crystal symmetry host exotic quantum
phenomena. Distinct from well-known Dirac and Weyl fermions,
structurally-chiral topological semimetals are predicted to host new types of
massless fermions characterized by a large topological charge, whereas such
exotic fermions are yet to be experimentally established. Here, by using
angle-resolved photoemission spectroscopy, we experimentally demonstrate that a
transition-metal silicide CoSi hosts two types of chiral topological fermions,
spin-1 chiral fermion and double Weyl fermion, in the center and corner of the
bulk Brillouin zone, respectively. Intriguingly, we found that the bulk Fermi
surfaces are purely composed of the energy bands related to these fermions. We
also find the surface states connecting the Fermi surfaces associated with
these fermions, suggesting the existence of the predicted Fermi-arc surface
states. Our result provides the first experimental evidence for the chiral
topological fermions beyond Dirac and Weyl fermions in condensed-matter
systems, and paves the pathway toward realizing exotic electronic properties
associated with unconventional chiral fermions.