The characterization of several isolates of a new lactic acid bacterium which produces a malty aroma is presented. Strains MX5R, MX5S, 674S, and 628S were asporogenous, catalase-negative, gram-positive, aciduric rods of varying length which did not reduce nitrate. The above strains also formed L(+)-lactic acid, produced 0.3 to 0.4% acidity in sterile milk, and grew at 7, 15, 25, and 32 C but not at 45 C. In addition, all strains produced limited amounts of 2 from gluconate, did not produce CO2 from glucose, required riboflavin and folic acid but not thiamine for growth, and demonstrated fructose-1,6-diphosphate aldolase (EC 4.1.2.13) activity. The guanine plus cytosine contents of deoxyribonucleic acid (DNA) extracted from representative strains MX5R and MX5S were 36.0 and 36.1 mol%, respectively. All isolates produced an extracellular nuclease which hydrolyzed ribonucleic acid as well as DNA. Like Streptococcus lactis subsp. maltigenes, all strains produced 2-methylpropanal, 2-methylpropanol, 3-methylbutanal, and 3-methylbutanol in skim milk and Trypticase soy broth. The characteristic malty aroma is most likely due to the production of the above aldehydes. On the basis of the data presented, the malty strains were placed in a new species, Lactobacillus maltaromicus, in the subgenus Streptobacterium. Strain MX5 (ATCO 27865) was designated the type strain of L. maltaromicus.