Technical Abstract: The circadian clock is a core molecular mechanism that allows organisms to anticipate daily environmental changes and adapt the timing of behaviors to maximize efficiency. In social insects, the ability to maintain the appropriate temporal order is important not only for the survival of individuals, but is also thought to improve colony efficiency and fitness. We used the newly sequenced fire ant (Solenopsis invicta) genome to characterize the first ant circadian clock and we describe a novel insect domain discovered on the recently identified clock gene, clockwork orange. Our results reveal that the fire ant clock is similar to the clock of the honeybee, a social insect with an independent evolutionary origin of sociality. Gene trees for the eight core clock genes, period, cycle, clock, cryptochrome, timeout, vrille, par domain protein 1 (pdp) & clockwork orange, show ant species grouping closely with honeybees and Nasonia as an outgroup to the social Hymenoptera. Expression patterns for these genes demonstrate that the ant clock functions similar to the honeybee clock, with period and cry-m mRNA levels increasing during the night and cycle and clockwork orange mRNAs cycling approximately anti-phase to period. In the fire ant only, vrille also appears to cycle. Gene models for five of these genes also parallel honeybee models. In particular, the single ant cryptochrome is an ortholog of the mammalian-type, rather than Drosophila-like protein. Additionally, we find a conserved VPIFAL C-tail region in clockwork orange shared by insects but absent in vertebrates. Overall, our characterization of the ant clock demonstrates that two socialinsects, ants and bees, share a similar, mammalian-like circadian clock. This study represents the first characterization of clock genes in an ant and is a key step towards understanding socially-regulated plasticity in circadian rhythms by facilitating comparative studies on the organization of circadian clockwork.