In recent years, there has been growing interest in fermentative hydrogen production using a variety of carbonaceous renewable resources as feedstocks. Among the various bacteria able to produce hydrogen through dark (i.e., non-photosynthetic) fermentation, particular attention has been given to Clostridium species. Extensive diversity has been observed among uncultured and as-yet uncharacterized clostridia. Experiments were conducted to assess the rate and yield of hydrogen production by two phylogenetically novel groundwater isolates belonging to the genus Clostridium. For comparison purposes, two Clostridium species previously studied in hydrogen production research, C. acetobutylicum (DSM 792T) and C. butyricum (DSM 10702T), were also studied.  Static batch experiments were carried out at 30ºC in anaerobic, buffered, glucose medium at pH 6.  Hydrogen, volatile suspended solids (VSS), and glucose concentrations were measured as a function of time following inoculation. Cumulative hydrogen production and VSS data were fitted to a modified Gompertz equation to calculate maximum rates of hydrogen production in terms of mL hydrogen produced per gram VSS per hour.  The environmental isolates exhibited higher maximum hydrogen production rates [248 and 245 mL·g^-1·h^-1] than C. acetobutylicum [216 mL·g^-1·h^-1] or C. butyricum [27.9 mL·g^-1·h^-1] under the conditions tested.  Hydrogen yield (mmol hydrogen produced per mmol of glucose consumed) was also higher for these novel isolates (1.40 ±0.04 and 1.35 ±0.06 mmol/mmol) than for C. acetobutylicum (1.20 ±0.18 mmol/mmol) or C. butyricum (0.73 ±0.10 mmol/mmol). Results demonstrate that previously uncharacterized hydrogen-producing clostridia hold promise for improved rate and yield in biological hydrogen production processes.