Technical Abstract: Fresh plant materials can be fermented and preserved as silage for cattle, but clostridia that deaminate amino acids increase pH. If silage pH rises, other spoilage microorganisms proliferate. Rod-shaped, anaerobic bacteria with spores were isolated from fresh alfalfa, fresh corn, and silages. Strain MD1 had the highest specific activity of amino acid deamination, and it was most closely related to Clostridium botulinum A and B. However, because MD1 did not produce a toxin, it was classified as Clostridium sporogenes. Washed cell suspensions of C. sporogenes MD1 had specific activities as great as 690 nmol ammonia mg protein-1 min-1, and this rate did not decrease until the pH was less than 4.5. Batch cultures of C. sporogenes MD1 did not initiate growth if the initial pH was less than 5.0, but continuous cultures (0.1 h-1 dilution rate) persisted until the pH of culture vessel was 4.6. When C. sporogenes MD1 was co-cultured with a bacteriocin producing Streptococcus bovis strain (HC5), ammonia production was greatly reduced. The ability of S. bovis HC5 to inhibit MD1 was pH-dependent. When the pH was 5.5 or less, MD1 could no longer be detected. These latter results support the idea that bacteriocin-producing bacteria may be used to improve silage quality.