  A scalar field obeying a Lorentz invariant higher order wave equation, is
minimally coupled to the electromagnetic field. The propagator and vertex
factors for the Feynman diagrams, are determined. As an example we write down
the matrix element for the Compton effect. This matrix element is algebraically
reduced to the usual one for a charged Klein-Gordon particle. It is proved that
the $ n^{th} $ order theory is equivalent to n independent second order
theories. It is also shown that the higher order theory is both renormalizable
and unitary for arbitrary n.
