Recently, Cardoso, Houri and Kimura constructed generalized ladder operators for massive Klein-\ud
Gordon scalar fields in space-times with conformal symmetry. Their construction requires a closed\ud
conformal Killing vector, which is also an eigenvector of the Ricci tensor. Here, a similar procedure is used\ud
to construct generalized ladder operators for the Klein-Gordon equation with a scalar curvature term. It is\ud
proven that a ladder operator requires the existence of a conformal Killing vector, which must satisfy an\ud
additional property. This property is necessary and sufficient for the construction of a ladder operator. For\ud
maximally symmetric space-times, the results are equivalent to those of Cardoso, Houri and Kimura