Monitoring animal populations is critical for the development of successful management strategies. This is especially important for populations that have been reintroduced to historic habitats, such as river otters (Lontra canadensis), which were reintroduced to Missouri during the 1980s and 1990s. We measured river otter relative abundance in southern Missouri using seasonal bridge-sign surveys on 15 study areas from 1996 to 1999. Otter sign was detected at 63% of the bridges during fall surveys and 82% of the bridges during spring surveys. Relative abundance during the fall 1996 survey (the only pre-harvest survey) was significantly higher (P  =  0.0014) than during the fall 1997 and fall 1998 surveys that followed the institution of a legal trapping season. Relative abundance based on spring surveys were not significantly different (P  =  0.196) over the 3 y. Repeat surveys conducted in the same year did not result in significantly different relative abundance in either fall 1997 (P  =  0.13) or spring 1997 (P  =  0.27). These results were similar to catch per unit effort (CPUE) estimates, but not entirely consistent. We recommend that managers conduct bridge-sign surveys during the same season and employ at least one other direct measure of the relative abundance (CPUE or mark-recapture) in order to monitor long-term relative abundance of otters.