Quantum fields in an expanding universe

We extend our analysis for scalar fields in a Robertson-Walker metric to th e electromagnetic and Dirac fields by the method of invariants. The issue o f the relation between conformal properties and particle production is re-e xamined and it is verified that the electromagnetic and massless spinor act ions are conformal invariant, while the massless conformally coupled scalar field is not. For the scalar field case it is pointed out that the violati on of conformal symmetry due to surface terms, although not influential for the equations of motion, does lead to effects in the quantized theory.