We conducted a mussel survey of the Jacks Fork, Missouri, an Outstanding National Resource Water, to document mussel diversity and distribution in the watershed, to determine if changes had occurred since a previous survey in 1982, and to relate observed changes to species traits. We surveyed mussels with timed tactile or visual searches at 28 sites during summer from 2017 to 2019 and compared our results with the 1982 survey. Catch per unit effort, number of live individuals, species richness, and diversity were significantly lower in 2017–19 than in 1982. The proportion of extirpation at the 11 resurveyed sites averaged 0.85 (range 0.50–1.00) among species, and the proportion of colonization was 0.0 for all species. There were no differences in the relative abundance of tribes, life-history strategies, or species of conservation concern between the two surveys, suggesting that the decline has occurred evenly across species, tribes, and life-history strategies. Ten species are possibly extirpated from the basin. Causes of the mussel decline in the Jacks Fork basin are unknown.