  We consider nonnegative solutions of a parabolic equation in a cylinder $D
\timesI$, where $D$ is a noncompact domain of a Riemannian manifold and $I
=(0,T)$ with $0 < T \le \infty$ or $I=(-\infty,0)$. Under the assumption [SSP]
(i.e., the constant function 1 is a semismall perturbation of the associated
elliptic operator on $D$), we establish an integral representation theorem of
nonnegative solutions: In the case $I =(0,T)$, any nonnegative solution is
represented uniquely by an integral on $(D \times \{0 \}) \cup (\partial_M D
\times [0,T))$, where $\partial_M D$ is the Martin boundary of $D$ for the
elliptic operator; and in the case $I=(-\infty,0)$, any nonnegative solution is
represented uniquely by the sum of an integral on $\partial_M D \times
(-\infty,0)$ and a constant multiple of a particular solution. We also show
that [SSP] implies the condition [SIU] (i.e., the associated heat kernel is
semi-intrinsically ultracontractive).
