The circadian rhythm is important to all organisms and plays a key role in physiology. Although much is known about the genetic regulation of the circadian clock in Drosophila melanogaster and in Mus musculus, less is known about the circadian clock in cnidarians, let alone in the lower Bilateria. This study attempts to fill the information gap between Cnidaria and higher metazoans by characterizing the core circadian clock of Isodiametra pulchra (Acoelomorpha). These animals live in between sand grains above the sulfide zone on low wave-action beaches, exposing them to a wide array of daily environmental changes. I. pulchra circadian core-clock proteins were identified by comparing known Drosophila and Mus orthologues. Final orthology assignments were chosen based on protein domain characteristics, sequence similarities, and phylogenetic analyses. By comparing known orthologues of clock proteins found in insects and vertebrates, three full sequences and two incomplete sequences were identified. I examined the mRNA expression of two of these proteins (IPUL1_3616 and IPUL1_2961) using a semi-quantitative RT_PCR approach. I found that IPUL1_3616 varied in a circadian fashion where expression was highest one hour before lights on, but IPUL1_2961 did not vary significantly in expression. This study is the first study of the circadian clock in a basal bilaterian
