  Isomers with three- and five-nucleon-hole configurations have been
established in $^{203}$Tl. These include newly identified levels with a
three-nucleon structure: {\it I}$^{\pi }$ = (15/2$^{-}$) with {\it T}$_{1/2}$ =
7.9(5) ns, and {\it I}$^{\pi }$ = (35/2$^{-}$) with {\it T}$_{1/2}$ = 4.0(5)
ns. In addition, five-quasiparticle states: {\it I}$^{\pi }$ = (39/2$^{-}$)
with {\it T}$_{1/2}$ = 1.9(2) ns, and {\it I}$^{\pi }$ = (49/2$^{+}$) with {\it
T}$_{1/2}$ = 3.4(4) ns have also been established. The previously determined
long-lived decay [{\it T}$_{1/2}$ = 6.6(3) $\mu $s from this work] is
associated with isomerism of the {\it I}$^{\pi }$ = (29/2$^{+}$) state. Levels
above this long-lived isomer have been identified through a delayed-prompt
coincidence measurement. Five-nucleon-hole states with excitation energies {\it
E}$_{x}$ $\approx $ 7 MeV have been established as well as possible octupole
excitations of the $^{208}$Pb core built on these levels. The level scheme of
$^{203}$Tl is extended up to {\it E}$_{x}$ $\approx $ 11 MeV with the inclusion
of 25 new transitions. Empirical and shell-model calculations have been
performed to aid in the description of the observed states which are found to
be predominantly of intrinsic character.
