A once common butterfly found only in Florida, Cyclargus thomasi bethunebakeri (Miami Blue), declined to near extinction during the 1990s. On 26 November 2006, we discovered this species on Boca Grande Key, 76 km west of Bahia Honda Key. Previously, the butterfly was only known to survive on Bahia Honda Key. Over the following 8-month period, we searched periodically for this butterfly on islands in the Great White Heron National Wildlife Refuge (GWHWNR) and the Key West National Wildlife Refuge (KWNWR). The Miami Blue was not found on 6 islands in GWHNWR, but was present at 8 of 10 areas in KWNWR. Seven of the occupied sites were on islands in the Marquesas Keys. The number of Miami Blue adults varied greatly by island and season, but were highest at the two largest colonies when Pithecellobium keyense (Blackbead; both a larval host and adult nectar plant) was flowering and producing new leaves. Eight other plant species were visited by adult Miami Blues for nectar. Threats to the newly discovered colonies are discussed.