Packing transitions in the lowest energy structures of Li clusters as a function of size have been identified via simulated annealing. For N≤21, the large p character of Li leads to unexpected ionic structures. At N25, a packing pattern based on interpenetrating 13-atom icosahedra and similar to that of Na and K appears. This pattern persists until at N=55, where another transition to a structure based on a Mackay icosahedron (MIC) occurs. For clusters of size 55 and 147, the optimized fcc structure representative of the bulk is still slightly higher in energy than the optimal MIC. © 1994 The American Physical Society.