SUMMARYThe zero set {z ∈ R 2 : (z) = 0} of an eigenfunction of the Schr odinger operatorwith an Aharonov-Bohm-type magnetic potential is investigated. It is shown that, for the ÿrst eigenvalue 1 (the ground state energy), the following two statements are equivalent: (I) the magnetic ux through each singular point of the magnetic potential A is a half-integer; and (II) a suitable eigenfunction associated with 1 (a ground state) may be chosen in such a way that the zero set of is the union of a ÿnite number of nodal lines (curves of class C 2 ) which emanate from the singular points of the magnetic potential A and slit the two-dimensional plane R 2 . As an auxiliary result, a Hardy-type inequality near the singular points of A is proved. The C 2 di erentiability of nodal lines is obtained from an asymptotic analysis combined with the implicit function theorem.