Lachnospira multiparus grew very well in an anaerobic 0.2% pectin medium, whereas Eubacterium limosum, which utilizes methanol, H(2)-CO(2), and lactate, did not. Cocultures of the two species grew at a somewhat more rapid growth rate than did L. multiparus alone and almost doubled the amount of growth as measured by optical density. In model experiments with cultures transferred once a day with a 2-day retention time, L. multiparus produced mainly acetate, methanol, ethanol, formate, lactate, CO(2), and H(2) from pectin. The coculture produced one-third more acetate, and butyrate and CO(2) were the only other significant end products. The results are discussed in relationship to microbial metabolic interactions and interspecies hydrogen transfer.