We use a full coupled-channels method to calculate collisional propert
ies of magnetically or optically trapped ultracold Li-6. The magnetic-
field dependence of the s-wave scattering lengths of several mixtures
of hyperfine states are determined, as are the decay rates due to exch
ange collisions. In one case, we find Feshbach resonances at B = 0.08
T and B = 1.98 T. We show that the exact coupled-channels calculation
is well approximated over the entire range of magnetic fields by a sim
ple analytical calculation.[S1050-2947(98)50703-2].