Let \(\{v_{\alpha }\}\) be a system of polynomial solutions of the parabolic equation \(a_{hk}\partial _{x_{h}x_{k}}u - \partial _t u =0\) in a bounded \(C^1\) -cylinder \(\Omega _{T}\) contained in \(\mathbb {R}^{n+1}\) . Here, \(a_{hk}\partial _{x_{h}x_{k}}\) is an elliptic operator with real constant coefficients. We prove that \(\{v_{\alpha }\}\) is complete in \(L^{p}(\Sigma ')\) , where \(\Sigma '\) is the parabolic boundary of \(\Omega _{T}\) . Similar results are proved for the adjoint equation \(a_{hk}\partial _{x_{h}x_{k}} u+ \partial _t u =0\) .