Transient spectral hole-burning experiments, using diode lasers, in minute external magnetic fields of < 16 mT are reported for the lower-energy E-2 <-- (4) A(2) spin-flip transition of NaMgAl(oxalate)(3)(-)9H(2)O/Cr(III). Ground and excited-state g factors can be accurately determined by an analysis of multiple side holes observed in the selective hole-burning experiments of the R-1( +/-3/2) and R-1( +/-1/2) transitions with Bparallel toc and Bperpendicular toc. The observed g factors, g(z)(ex)(R-1) = 1.38 +/- 0.04, g(x)(ex)(R-1) = 2.00 +/- 0.08, and g(y)(ex)(R-1) = 1.82 +/- 0.08, imply a large variation from trigonal behavior that is caused by a low-symmetry component of the ligand field. The zero-field hole widths in a 0.5% crystal are similar to19 +/- 5 MHz and 23 +/-5 MHz for holes at the low-energy edge of the R-1( +/-1/2) line and the high-energy edge of the R, ( +/-3/2) transitions, respectively. Upon application of a magnetic field of Bparallel toc = 7 mT, a substantial narrowing of the hole width is observed for the R-1 ( +/-1/2) transition, with an upper limit for the homogeneous line width of similar to1 MHz. The present work illustrates the potential of diode lasers in the determination of the magnetic properties of transition-metal complexes by transient spectral hole-burning experiments in low magnetic fields.