Systematic monitoring of nesting and fledging of American White Pelicans (Pelecanus erythrorhynchos) breeding on the Molly Islands In Yellowstone Lake was carried on for 20 years between 1966 and 1987. The mean number of pelicans fledged during those years was 214 (±196 SD). In five of those years the number of pelicans fledged was at or near zero. In six other years, the number of young pelicans fledged ranged from 302-650, with four of those years producing record numbers of successfully fledged young. Maximum seasonal levels of Yellowstone Lake reaching 1.6 m or more produced complete or near complete reproductive failures. Conversely, years with maximum lake levels never reaching 1.4 m were most likely to have very good to outstanding fledging success. Also, numbers of days when air temperatures of -8.8° C or colder and daily temperature minimums of -16.6° C, as well as the duration of ice cover on Yellowstone Lake were important modifiers of the numbers of pelicans fledged. Unexpected crustal deformations adjacent to and within the Yellowstone Lake basin induced flooding, representing serious geologic threats to the long-term well being of the Molly Island pelican colony. A baseline breeding population of 546 (± 183 SD) adult pelicans was calculated to have supported this Molly Island colony. Also, the percentage of population change in the computed annual life cycle for 1966-87 was +5%.