  We consider the evolution of an isentropic thermal instability in the atomic
zone of a photodissociation region (PDR). In this zone, gas heating and cooling
are associated mainly with photoelectric emission from dust grains and
fine-structure lines ([\ion{C}{ii}] 158, [\ion{O}{i}] 63, and [\ion{O}{i}] 146
{\micron}), respectively. The instability criterion has a multi-parametric
dependence on the conditions of the interstellar medium. We found that
instability occurs when the intensity of the incident far-ultraviolet field
$G_0$ and gas density $n$ are high. For example, we have $3\times10^3<G_0<10^6$
and $4.5\times10^4<n<10^6$ {\cmc} at temperatures $360 <T<10^4$ K for typical
carbon and oxygen abundances $\xi_{\rm C}=1.4\times10^{-4}$ and $\xi_{\rm
O}=3.2\times10^{-4}$. The instability criterion depends on the relation between
$\xi_{\rm C}$ and $\xi_{\rm O}$ abundances and line opacities. We also give
examples of observed PDRs where instability could occur. For these PDRs, the
characteristic perturbation growth time is $t_{\rm inst}\sim10^3$ -- $10^4$ yr
and the distance characterizing the formation of secondary waves is
$L\sim10^{-3}$ -- $5\times10^{-2}$ pc. For objects that are older than $t_{\rm
inst}$ and have sizes of the atomic zone larger than $L$, we expect that
instability influences the PDR structure significantly. The presence of
multiple shock waves, turbulent velocities of several kilometers per second and
inhomogeneities with higher density and temperature than the surrounding medium
can characterize isentropic thermal instability in PDRs.
