The effects of weak magnetic fields (< 16 mT) in persistent non-photochemical spectral hole-burning of the R-1-line (E-2 <--(4)A(2) transition) in NaMgAl(oxalate)(3) . 9H(2)O : Cr(III) are reported. The observed hole patterns can be quantitatively rationalized with excited state g-factors, g(z) (ex) = 1.395 +/- 0.015, g(x)(ex) = 2.00 +/- 0.08, and g(y)(ex) = 1.82 +/- 0.08. The deviation from trigonal behavior implies the presence of a significant low symmetry ligand-field. Measuring the holeburning spectrum in a magnetic field after burning in zero field and vice versa yields the same hole pattern. The latter provides a persistent memory of magnetic fields. Zeeman effects in spectral holes may also be used to stabilize and tune the frequency of single-mode semiconductor lasers. (C) 2003 Elsevier Science B.V. All rights reserved.