Crystal structures were detd. for trimethylboron, BMe3, and for a new polymorph of trimethylgallium, GaMe3; the crystal structure of trimethylthallium, TlMe3, was redetd. The BMe3 crystal structure represents a new structural type for the group 13 tri-Me derivs. in the solid state. In contrast to its heavier analogs, it consists of layers contg. only very weakly interacting BMe3 mols. GaMe3 forms a ladder-like pseudo-polymer via long Ga-to-Me intermol. interactions with Ga...C distances in the range 3.096(3)-3.226(4) .ANG.. This is compared with a recently reported crystal structure of a polymorph, which, like InMe3 and TlMe3, was characterized by the formation of pseudo-tetramers. The effects of crystn. and secondary interactions were analyzed by comparison with related crystallog., gas-phase electron diffraction, and spectroscopic studies of these and other tri-Me derivs. of the group 13 elements. The energetic differences between polymorphs of BMe3, GaMe3, and InMe3 were explored by plane wave DFT calcns. The energy differences between the BMe3-like layered structure and the InMe3-like pseudo-tetrameric structure are -1.7, +3.6, and +10.4 kJ mol-1 for BMe3, GaMe3, and InMe3, resp.