  This work is a galoisian study of the spectral problem $L\Psi=\lambda\Psi$,
for algebro-geometric second order differential operators $L$, with
coefficients in a differential field, whose field of constants $C$ is
algebraically closed and of characteristic zero. Our approach regards the
spectral parameter $\lambda$ an algebraic variable over $C$, forcing the
consideration of a new field of coefficients for $L-\lambda$, whose field of
constants is the field $C(\Gamma)$ of the spectral curve $\Gamma$. Since
$C(\Gamma)$ is no longer algebraically closed, the need arises of a new
algebraic structure, generated by the solutions of the spectral problem over
$\Gamma$, called "Spectral Picard-Vessiot field" of $L-\lambda$. An existence
theorem is proved using differential algebra, allowing to recover classical
Picard-Vessiot theory for each $ \lambda = \lambda_0 $. For rational spectral
curves, the appropriate algebraic setting is established to solve
$L\Psi=\lambda\Psi$ analitically and to use symbolic integration. We illustrate
our results for Rosen-Morse solitons.
