We consider semilinear parabolic equations on unbounded domains in IR2
or IR3 with forcing polynomial nonlinearities of the form g(u) = au(p
) + bup(p+1) + ... with a > 0 and p greater than or equal to 2. It is
shown that the zero solution of the induced semiflow is positively uns
table, provided p = 2 and the domain contains arbitrarily large sphere
s. If p > 2 and if g possesses a positive root, then the zero solution
is positively stable.