The objective of this study was to assess fermentation product, growth rate and growth yield responses of Selenomonas ruminantium HD4 to limiting and non-limiting ammonia concentrations. The ammonia half-inhibition constant for S. ruminantium in batch culture was 296 mM. Cells were grown in continuous culture with a defined ascorbate-reduced basal medium containing either 0.5, 5, 25, 50, 100 or 200 mM NH4Cl and dilution rates were 0.07, 0.14, 0.24 or 0.40 h(-1). Ammonia was the growth-limiting nutrient when 0.5 mM NH4Cl was provided and the half-saturation constant was 72 mu M Specific rates of glucose utilization and fermentation acid carbon formation were highest for 0.5 mM NH4Cl. Lactate production (moles per mole of glucose disappearing) increased at the fastest dilution rate (0.40 h(-1)) for 5.0 mM NH4Cl while acetate and propionate decreased when compared to slower dilutions (0.07 and 0.14 h(-1)). Lactate production remained low while acetate and propionate remained high for all dilution rates when NH4Cl concentrations were 25 mM or greater. Yield (Y-Glc and Y-ATP) were nearly doubled when NH4Cl was increased from 0.5 mM (25.1 g cells/mol glucose used and 13.9 g cells/mol ATP produced respectively) to the higher concentrations. Y-Glc was highest at 25 mM and 50 mM NH4Cl (48.2 cells/mol and 43.1 cells/mol respectively) as was Y-ATP (23.2 cells/mol and 20.8 cells/mol respectively). Y-NH3 was highest at the lowest NH4Cl concentration. The maximal fermentation product formation rate occurred at a growth-limiting ammonia concentration, while maximal glucose and ATP bacterial yields occurred at non-growth-limiting ammonia concentrations. Given the growth response of this ruminal bacterium, it is possible that maximization of ruminal bacterial yield may necessitate sacrificing the substrate degradation rate and vice versa.