Partial conservation of seniority and nuclear isomerism.

We point out the possibility of the partial conservation of the seniority quantum number when most eigenstates are mixed in seniority but some remain pure. This situation occurs in nuclei for the g(9/2) and h(9/2) shells where it is at the origin of the existence of seniority isomers in the ruthenium and palladium isotopes. It also occurs for f bosons.