An analogue of the Sturm oscillation theory of the distribution of the zeros of eigenfunctions is constructed for the problem
 | (*) |
on a spatial network 
(in other terms, is a metric graph, a CW complex, a stratified locally one-dimensional manifold, a branching space, a quantum graph, and so on), where 
is the family of boundary vertices of . At interior points of the edges of the quasi-derivative 
has the classical form 
, and at interior nodes it is assumed that
where the summation is taken over the edges 
incident to the node 
and, for an edge , 
stands for the `endpoint' derivative of the restriction 
of the function 
to . Despite the branching argument, which is a kind of intermediate type between the one-dimensional and multidimensional cases, the outward form of the results turns out to be quite classical. The classical nature of the operator 
is clarified, and exact analogues of the maximum principle and of the Sturm theorem on alternation of zeros are established, together with the sign-regular oscillation properties of the spectrum of the problem (*) (including the simplicity and positivity of the points of the spectrum and also the number of zeros and their alternation for the eigenfunctions).
