  The scalar field theory with higher derivatives is considered in the first
order formalism. The field equation of the forth order describes scalar
particles possessing two mass states. The first order relativistic wave
equation in the 10-dimensional matrix form is derived. We find the
relativistically invariant bilinear form and corresponding Lagrangian. The
canonical energy-momentum tensor and density of the electromagnetic current are
obtained. Dynamical and non-dynamical components of the wave function are
separated and the quantum-mechanical Hamiltonian is found. Projection operators
extracting solutions of field equations for definite energy and different mass
states of particles are obtained. The canonical quantization of scalar fields
with two mass states is performed, and propagators are found in the formalism
considered.
