Most of the fish in Laos are migratory. Many species migrate seasonally and can travel remarkably long distances to reach critical habitats or spawning grounds. Ensuring connectivity between these habitats is key to maintaining fish abundance and biodiversity. Small weirs, designed to divert water for irrigation or electrical purposes, can impede or obstruct fish passage, and can also cause substantial hydrological and ecological changes in a river due to the change in flow. FISHBIO evaluated an irrigation weir on the Nam Et River (Et District, Houaphanh Province) in northern Laos. The weir, known as the Keng Lit Waterfall, was constructed in 2003 to provide irrigation for nine nearby villages. This weir was designed with a fish ladder; however, the ladder is not effective at facilitating passage for most fish species. The design of fish passages is essential for successful upstream and downstream migration. In complex, multi-species ecosystems, it is difficult to design passages that are accessible to all species. FISHBIO conducted a fish passage assessment to determine the effectiveness of the ladder. We considered the target species at the location and focused on design elements such as the fish passage structure, slope, flow velocity, and location of the entrance and exit, as these all influence which species are able to navigate the passageway.