Phenology is an important dimension of vertebrate reproductive strategies. Characterizing a species' reproductive phenology is a fundamental step in directing studies of influences on reproduction within and among populations throughout its range. Eastern Diamond-Backed Rattlesnakes (Crotalus adamanteus; EDBs) are endemic predators of the imperiled longleaf pine (Pinus palustris) ecosystem of the southeastern United States, and the species currently is under review for listing under the Endangered Species Act. Conservation and management of this imperiled species will greatly benefit from research on its basic biology, and particularly from information that will assist management for adult survival and population viability. Here, we present long-term data on EDB breeding and reproductive phenology from several populations in the South Carolina Coastal Plain. We summarized the phenology of EDB breeding and reproduction using observations of free-ranging, radiotelemetered rattlesnakes in South Carolina from 1997 to 2011. Breeding activity occurred from 10 July to 14 October, with a mean of 28 August. There was one outlying observation of spring breeding (18 March). Females moved to birthing sites as early as 26 June and as late as 28 August, with a mean of 15 August. Neonates were born around 31 August, with birth dates between 10 August and 14 September. We also noted six unique observations of apparent clutch size: 8, 10, 11, 11, 12, and 13 neonates. This information will contribute both to our current knowledge and conservation efforts of EDBs, and also facilitate future long-term comparative investigations throughout their geographic range.