  Matter fields in the MSSM are chiral supermultiplets in fundamental (or
singlet) representations of the standard model gauge group. In this paper we
introduce chiral superfields in the adjoint representation of $SU(3)_C$ and
study the effective field theory and phenomenology of them. These states are
well motivated by intersecting D-brane models in which additional massless
adjoint chiral supermultiplets appear generically in the low energy spectrum.
Although it has been pointed out that the existence of these additional fields
may make it difficult to obtain asymptotic freedom, we demonstrate that this
consideration does not rule out the existence of adjoints. The QCD gauge
coupling can be perturbative up to a sufficiently high scale, and therefore a
perturbative description for a D-brane model is valid. The full supersymmetric
and soft SUSY breaking Lagrangians and the resulting renormalization group
equations are given. Phenomenological aspects of the adjoint matter are also
studied, including the decay and production processes. The similarity in gauge
interaction between the adjoint fermion and gluino facilitates our study on
these aspects. It is found that these adjoint multiplets can give detectable
signals at colliders and satisfy the constraints from cosmology.
