A mark-release-recapture study of the federally-listed Smith's Blue butterfly, Euphilotes enoptes smithi, was conducted in 1986 at Marina State Beach (Monterey County, CA) in coastal sand dune habitat. Demographic parameters estimated included daily and generation population numbers, residency, vagility, and emigration rates. Manly-Parr and Jolly-Seber analyses indicated an estimated 2,657 to 5,875 individuals comprised the 1986 generation, with average residence from 6.7 to 10.1 days (maximum 20 days). Average movements between consecutive observations on different days were 95.1 m for males and 91.0 m for females, while the longest lifetime dispersal observed was 1,230 m by a female. Emigration rates from areas of low quality habitat were as much as 8x greater than those from high quality habitat. Between 1997 and 2017 this butterfly was monitored yearly at three neighboring sand dune remnants in Sand City, CA to determine annual generation sizes. Transect counts of adults conducted during the 21 flight seasons, in conjunction with the frequencies of observed residencies from the 1986 study, indicated estimated generation numbers from 360 to 2,151 for the three sites combined. The observed year-to-year fluctuations in generation sizes were strongly correlated with annual numbers of flowerheads of this butterfly's Eriogonum foodplants, which in turn was correlated with annual precipitation.