The diatomic aluminum and gallium anion radicals (Al-2(-) and Ga-2(-)) have been studied by electron spin resonance (ESR) as isolated ions in neon matrices near 4 K. Apparently no previous examples of purely metallic anions have been studied by the rare gas matrix isolation trapping technique with any type of spectroscopic method. Pulsed laser ablation of the metals was the experimental generation method which has enabled the first direct characterization of the ground electronic states of these anions. The X (4)Sigma assignment agrees with previous interpretations of photoelectron spectroscopic results. The analysis of fine structure and nuclear hyperfine interactions has yielded a detailed description of the electronic structure in these seven-valence electron anion radicals, with comparisons to other isovalent X (4)Sigma radicals presented. Experimental values of the nuclear hyperfine interactions (A tensors) were compared with theoretical results obtained from various ab initio computational methods, including configuration interaction and density functional theory. Reasonable agreement between these experimental results and theoretical predictions was observed.