  For the \kappa-symmetry gauge fixed superstring action in AdS_5 x S^5 we
consider the fermionic fluctuations over a circular bosonic string background
with two angular momenta and two winding numbers in S^5. The SU(2)-type
redefinitions of fermionic fields and the first-string limit generate a
truncated fermionic action for the SU(1|2) sector. It is expressed in a
two-dimensional Lorentz-invariant form of a massive Dirac fermion and the
plane-wave spectrum for the fermionic excitations is derived. The fermionic
spectrum for the SU(2|2) sector is also analyzed.
