  If $(G,V)$ is a multiplity free space with a one dimensional quotient we give
generators and relations for the non-commutative algebra $D(V)^{G'}$ of
invariant differential operators under the semi-simple part $G'$ of the
reductive group $G$. More precisely we show that $D(V)^{G'}$ is the quotient of
a Smith algebra by a completely described two-sided ideal.
