Flame synthesis of single-walled carbon nanotubes and nanofibers is demonstrated using a pyrolysis flame configuration. Fe reacts preferentially with CO/H-2/He mixtures to produce single-walled nanotubes (SWNTs). In contrast, Ni reacts preferentially with C2H2/H-2/He mixtures to yield nanofibers. Both catalyst metals exhibit a marked size dependent reactivity towards these reactant gas mixtures. The yarmulke mechanism and a carbon solvation/diffusion/ precipitation account for the different catalyzed products, SWNTs and nanofibers. respectively, Consequences of the size dependent reactivities of Fe and Ni nanoparticles and the respective growth mechanisms for the SWNTs and nanofibers are discussed.