The aim of this work was to study the metabolic and microbial community dynamics during dark co-fermentation of 80% tequila vinasse and 20% nixtamalization wastewater (w/w). Batch co-fermentations were performed in a 3-L well-mixed reactor at 35 degrees C and pH 5.5. In correspondence to Illumina MiSeq sequencing and reactor monitoring, changes in metabolites and microbial communities were characterized by three main stages: (i) a first stage during which lactate and acetate producers dominated and consumed the major part of fermentable carbohydrates, (ii) a second stage in which lactate and acetate were consumed by emerging hydrogen-producing bacteria (HPB) in correlation with bioH(2) (100 NmL/L-h or 1200 NmL/L-reactor) and butyrate production, and (iii) a third stage during which non-HPB outcompeted HPB after bioH(2) production ceased. Altogether, the results of this study suggest that cooperative interactions between lactate producers and lactate- and acetateconsuming HPB could be attributed to lactate- and acetate-based cross-feeding interactions.