In a recent experiment [Weinstein et al., Nature 395, 148 (1998)] we magnetically trapped 108 ground-state calcium monohydride molecules, CaH(X 2Σ,v″=0, J″=0). The molecules were prepared by laser ablation of a solid sample of CaH2 and loaded via thermalization with a cold (<1 K) 3He buffer gas. The magnetic trap was formed by superconducting coils arranged in the anti-Helmholtz configuration. The detection was done by laser fluorescence spectroscopy excited at 635 nm (in the B 2Σ,v′=0−X 2Σ,v″=0 band) and detected at 692 nm (within the B,v′=0−X,v″=1 band). Both a photomultiplier tube and a CCD camera were used. Due to the thermalization of molecular rotation, only a transition from the lowest rotational state could be detected at zero field, N′=1, J′=3/2←N″=0, J″=1/2. In the magnetic field this rotational transition splits into two features, one shifted towards lower and one towards higher frequencies. The measured shifts are linear in field strength and indicate a small difference (0.02 μB) in the magnet…