Please cite this article in press as: N. Xafenias, et al., Electrochemical startup increases 1,3-propanediol titers in mixed-culture glycerol fermentations, Process Biochem (2015), http://dx. a b s t r a c tIn this study we investigated the use of electric potential to bioelectrochemically ferment glycerol, a cheap by-product of biodiesel production, into valuable 1,3-propanediol (1,3-PDO). The 1,3-PDO production rates were increased up to 6 times in electrofermentations, compared to non-electrochemical fermentations, and high concentrations up to 42 g 1,3-PDO/l were achieved in fed-batch mode. Extensive growth of the well-known 1,3-PDO producers Clostridiaceae (55-57%) was observed when an appropriate potential (−1.1 V vs. SHE) was constantly applied since the start. Potential propionate producers (Veillonellaceae) were also among the dominant families (20-21%); however, surprisingly enough, propionate production was not observed. On the contrary, Clostridiaceae were absent, Veillonellaceae dominated (56-72%), and propionate was produced when electric potential was not sufficient for current production since the beginning. In all cases, glycerol consumption ceased and electrocatalytic activity was lost when we replaced the biofilm electrodes with electrodes lacking a biofilm, clearly demonstrating that glycerol electrofermentation was mostly supported by the bacteria located in the biofilm. In the nonelectrochemical systems the performance and the titers achieved were poor; only 18 g 1,3-PDO/l was achieved in more than twice the time, and lactate producing Lactobacillaceae became dominant.