Fish passes are commonly installed to partially reinstate river connectivity for fish species. They are designed according to the hydrological context and the biological features of the target species. Vertical slot fishways (VSF) still need a design optimization for small fish species, potentially relying on a better knowledge of fish behaviour inside the pools. This work studied the behaviour of two common fish species in France rivers: brown trout (Salmo trutta L.) and chub (Squalius cephalus L.) while passing through a VSF model. Fish movements were recorded by video tracking and analyzed following two steps: (i) global fish passages and (ii) fish behaviour inside one pool (trajectories, use of resting zones, movement patterns, position, and orientation before a successful upstream move). The mean total efficiencies (i.e. number of individuals that managed to pass and stay upstream over the total number of individuals studied) were 23.6 and 35.7% for trout and chub, respectively. Two behaviours were observed: direct passages in the upstream pool and passages with stops in one or more "resting" zones. Direct passages induced short transit times (several seconds) while a resting behaviour induced longer times (up to several minutes). A preferential resting zone, where flow conditions were the least constraining, was highlighted. Finally, the distance of fish from the slot was identified as a key parameter to ensure a successful passage. These results highlight the importance of the presence of resting zones and their hydraulic conditions for small fish passage success in VSF.