p53 and its main negative regulator, Mdm2, are key players in mammalian cancer development. Activation of the transcription factor p53 through DNA damage or other stresses can result in cell cycle arrest, apoptosis, or both. Because of the absence of characterized p53 signaling in zebrafish (Danio rerio), we have studied the roles of Mdm2 and p53 in zebrafish by generating early embryonic knockdowns and examined the involvement of p53 in DNA damage-induced apoptosis. p53-deficient embryos, induced by injection of antisense morpholinos, were morphologically indistinguishable from control embryos, when unperturbed, whereas Mdm2 knockdown embryos were severely apoptotic and arrested very early in development. Double knockdowns showed that p53 deficiency rescued Mdm2-deficient embryos completely, similar to observations in mice. p53 deficiency also markedly decreased DNA damage-induced apoptosis, elicited by ultraviolet irradiation or by the anti-cancer compound camptothecin. p21/Waf/Cip-1 appeared to be a downstream target of zebrafish p53, as revealed relative p21 mRNA levels determined via TaqMan analysis. In contrast to mammals, zebrafish may regulate p53 activity by using an internal polyA signal site. We conclude that zebrafish represents a promising model organism for future compound-based and genetic screens and believe that it will help to identify and characterize new anticancer drugs and new targets for cancer treatment.