Microbial anodes were formed under constant polarization either free in solution or inside channels of 1 mm or 5 mm height. Both the 5 mm and 1 mm high channels allowed the colonization of electrodes over several centimetres of penetration depth. The channel impacted the evolution of the biofilm along the electrode and limited the maximum current density to 5.4 A.m(-2), whatever the height, while 13.1 A.m(-2) was achieved with the free bioanodes. Several effects related to the channel configuration can be explained by a theoretical analysis of mass transfers. Nevertheless, other observations, such as the stabilizing effect of the channels on the long-term current, the time of 2 days that was always sufficient for the electrodes reach the maximum current, and the decoupling between current production and biofilm development, remain difficult to explain and open up exciting questions for future research on the mechanisms of electroactive biofilm formation. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.