A protein present in extracts of the dinoflagellate Gonyaulax that was capable of binding an antibody directed against the conserved Cdc2 kinase epitope EGVPSTAIREISLLKE was characterized by Western blot analysis and DNA sequencing and shown not to encode a Cdc2 kinase. The amount, size, and isoelectric point of the immunoreactive species were invariant over a 24-hour period (encompassing S and M phases), and the DNA sequence of a cDNA isolated by immunologic screening showed that no conserved kinase regions were present in the deduced amino acid sequence. A method based on polymerase chain reaction (PCR), using primers designed from conserved regions in the Cdc2 kinases, was also unsuccessful in isolating a cdc2 gene homologue, although other kinases were identified.