Basic helix-loop-helix (bHLH) transcription factors play significant roles in multiple biological processes in metazoan cells. In recent work, we showed that three orthologous HLH families, pearl, amber, and peridot, have apparently been lost in the Drosophila melanogaster, Caenorhabditis elegans, and Homo sapiens lineages. To further address the gain and loss of bHLH proteins during bilaterian evolution, we examined the genome of the pearl oyster, Pinctada fucata, which has recently been sequenced. We characterized the putative full set 65 bHLH genes and showed that genes previously categorized into the orthologous family PTFb, actually fall into two distinct orthologous families, 48-related-1 and 48-related-2. We also identified a novel orthologous family, clockwork orange. Based on these newly identified orthologous family members and on orphan bHLH factors, we propose that genes encoding bHLH factors in bilaterians are not as evolutionarily stable as previously thought.