SUMMARY The identification and functional studies of DM domain-containing proteins Doublesex, MAB-3, and DMRT1 indicated that flies, nematodes, and humans share at least some of the molecular mechanisms of sex determination. We identified a gene, AmDM1, from the coral Acropora millepora that encodes a homologous DM domain-containing protein. Molecular analyses show that the AmDM1 primary transcript is processed to generate four different messenger RNAs. Alternative use of two polyadenylation sites produces transcripts that vary only in the 3′ untranslated regions, whereas alternative splicing generates transcripts with and without the region coding for the DM domain. All the transcripts include a second motif, the DMA domain, which is found in a number of other proteins containing a DM domain. Hermaphroditic A. millepora differentiates sexual cells seasonally before the spring spawn, and Northern blot analysis shows that the AmDM1 transcripts are present at higher levels during sexual differentiation. The non-DM domain-containing messages are also present at significant levels in late embryos, but DM domain transcripts are extremely rare at this stage. These data suggest that the association of DM domain proteins and sexual determination or differentiation predates the separation of the Cnidaria from the rest of the Metazoa.