Large colonies of nesting ciconiiform birds have recently become an important tourist attraction in the Brazilian Pantanal. Visits to these colonies are uncontrolled and there is growing concern about disturbance to the breeding birds. In 1998, an experiment was conducted using different treatments within a single, large, heavily visited colony to assess the effects of different kinds of tourist viewing on the breeding behavior and nesting success of the Wood Stork (Mycteria americana). Pedestrian viewing, when more than 75 m from the nearest active nest, produced no detectable change in breeding success or behavior. However, boats passing within 20 m of nests were associated with significant changes in the time budgets of incubating adult storks. Most breeding failure occurred prior to hatching and was significantly higher in the boat-treatments in comparison with controls. It is suggested that the elevated level of nest failure observed in the boat-treatment group is evidence of biologically significant disturbance. These conclusions are supported by observations taken in 2000 when boats were excluded from the colony. Behavioral responses at the time of disturbance are typically used as indicators of effects when determining safe approach distances for birds. However, in this study, behavior during disturbances was unrelated to the eventual effect of disturbance. These findings have been incorporated into suggestions for the measurement of disturbance and management of tourist visits to bird colonies.