  The electronic structure of interstitial hydrogen in a compound semiconductor
FeS$_2$ (naturally $n$-type) is inferred from a muon study. An implanted muon
(Mu, a pseudo-hydrogen) forms electronically different defect centers discerned
by the hyperfine parameter ($\omega_{\rm hf}$). A body of evidence indicates
that one muon is situated at the center of an iron-cornered tetrahedron with
nearly isotropic $\omega_{\rm hf}$ (Mu$_{\rm p}$), and that the other exists as
a diamagnetic state (Mu$_{\rm d}$, $\omega_{\rm hf}\simeq 0$). Their response
to thermal agitation indicates that the Mu$_{\rm d}$ center accompanies a
shallow level (donor or acceptor) understood by effective mass model while the
electronic structure of Mu$_{\rm p}$ center is more isolated from host than
Mu$_{\rm d}$ to form a deeper donor level. These observations suggest that
interstitial hydrogen also serves as an electronically active impurity in
FeS$_2$. Based on earlier reports on the hydrogen diffusion in FeS$_2$,
possibility of fast diffusion for Mu$_{\rm p}$ leading to formation of a
complex defect state (Mu$^*_{\rm d}$, $T\le 100$ K) or to motional narrowing
state (Mu$^*_{\rm p}$, $T\ge 150$ K) is also discussed.
