Isoforms of p53 have been described in both zebrafish and human systems based on sensitive analysis of RNA using PCR-based methods. Despite consistent evidence of the existence of these isoforms at the RNA level it has been difficult to detect the endogenous proteins in a physiological setting. In the zebrafish we have previously shown that the mRNA encoding the ∆113p53 is abundantly induced in whole embryos following induction of the p53 response by radiation, CDK inhibitors and chemotherapeutic drugs. Using a set of monoclonal antibodies raised against different domains of ZFp53 we now show for the first time clear evidence for the controlled expression of a truncated form of the ZFp53 protein, ∆113p53. The protein is present at very low levels but is induced by transcriptionally active full-length ZFp53 following the exposure of zebrafish embryos to the CDK inhibitor roscovitine. Induction of the protein is completely ZFp53 dependent and morpholinos that specifically block the expression of endogenous D113p53 protein selectively enhance the expression of some but not all ZFp53 responsive genes. We map the p53 response elements in the ∆113p53 promoter using functional assays and identify an region at 1593-1612 in intron 4 of ZFp53, as being crucial in the full-length promoter. Thus the endogenous D113p53 protein, which oligomerises with the full-length ZFp53 protein, can act as a selective dominant negative inhibitor of the ZFp53 response, creating a distinct feedback response that varies the nature of the p53 response over time after exposure to an inducing signal.