There is interest in novel fibers as potential prebiotics for new and reformulated food products. Two konjac glucomannan (KGM) hydrolysates were developed by enzymatic hydrolysis with (KGMH I) or without (KGMH II) mechanical shear pre-treatment. These were characterized and evaluated as fermentation substrates using five lactobacilli and three bifidobacteria. Enzymatic treatment of native KGM reduced the average molecular weights of supernatant and pellet by ∼3-fold. Additional mechanical shear pre-treatment further reduced supernatant and pellet molecular weights by 5% and 35%, respectively. We postulated that pulverized and depolymerized short-chain KGM would better promote the growth of lactobacilli and bifidobacteria. Most lactobacilli fermented KGM hydrolysates. Lactobacillus acidophilus and Lactobacillus plantarum fermented KGMH I and II better than they fermented inulin. Overall, bifidobacteria were not strong fermenters of KGM hydrolysates. Both pulverization and enzymatic depolymerization significantly affected KGM molecular weight, suggesting that human gastrointestinal bacteria can utilize KGM hydrolysates with reduced weights.