It is our thesis that members of the Stephen Long Expedition of 1819-20 completed the first biodiversity inventory undertaken in the United States at their winter quarters, Engineer Cantonment, Missouri Territory, in the modern state of Nebraska. This accomplishment has been overlooked both by biologists and historians, but it should rank among the most significant accomplishments of the expedition. The results of this inventory allow us to evaluate the environmental, faunal, and floral changes along the Missouri River in the intervening nearly 190 years. The historical records form a visual image of a dynamic riverine system in which a highly meandering river flows through a wide valley filled with oxbows, palustrine wetlands, and scattered groves of trees. This system has now been modified to a channelized river with the surrounding wetlands drained and converted to agricultural and municipal purposes. The suppression of prairie fires and the adoption of irrigation practices have promoted the growth of trees and other woody vegetation. The city of Omaha and its suburbs are expanding and encroaching on the site from the south and west. At least three taxa recorded at the site have become extinct-Ectopistes migratorius (passenger pigeon), Conuropsis carolinensis (Carolina parakeet), and Canis lupus nubilus (plains subspecies of the gray wolf)-and several more have been extirpated from the region. For mammals, the data indicate that nine species of the 1819-20 fauna have been lost, and two species have been added, thus resulting in a net loss of seven species. These changes represent a net loss of 15% of the mammalian biodiversity originally present in the Engineer Cantonment area. The species richness estimator for Engineer Cantonment in 1819-20 is 403 for vertebrates, insects, snails, and plants, but it is clear that this number is extremely low, because plants were not thoroughly surveyed by the expedition and only a small fraction of the insects were collected