Abstract. We study the behaviour of motivic zeta functions of prehomogeneous vector spaces under castling transformations. In particular we deduce how the motivic Milnor fibre and the Hodge spectrum at the origin behave under such transformations. §1. Introduction 1.1. Let us recall the classification of irreducible regular prehomogeneous vector spaces due to Sato and Kimura [18]. A prehomogeneous vector space over a field k [18] consists of the datum (G, X) of a connected linear k-algebraic group G together with a linear representation ρ of G on a finite dimensional affine space X = A m k over k having a Zariski dense G-orbit. The complement of the Zariski dense G-orbit is called the singular locus of the prehomogeneous vector space. When X is an irreducible G-module, one says the prehomogeneous vector space (G, X) is irreducible.One may find a general definition of regular prehomogeneous vector spaces in [18], which, when the group G is reductive, is equivalent to the condition that the singular locus is an hypersurface. From now on, except for Section 7, we shall always assume the group G to be reductive. When (G, X) is an irreducible regular prehomogeneous vector space, the singular locus is an irreducible hypersurface f = 0 in X.In the theory of Sato and Kimura, a fundamental role is played by castling transforms, which are defined as follows.Let G be a connected reductive k-algebraic group and ρ be a linear representation of G on A m k . Write m = r 1 + r 2 . Then set X 1 = M m,r 1 and let G × SL r 1 act on X 1 by ρ 1 (g, g 1 )x 1 = ρ(g)x 1 t g 1 .