The widespread expression of the Xenopus homeobox gene Xhox3 in zebrafish was performed by microinjection of synthetic Xhox3 mRNA into uncleaved fertilized eggs and resulted in embryos displaying varying degrees of anterior-posterior (A-P) axis disruption. The phenotype observed was dose-dependent and showed anomalies, mainly in neural keel development, from microphthalmia to acephaly. Injection of 5 pg to 10 pg of mRNA caused a range of phenotypes in prim 5 stage embryos from normal to acephalic. Anomalies have been categorized according to an index of axis deficiency (Zf-IAD). We further characterized the axis disturbance by analyzing the expression of two genes implicated in axis formation: engrailed (eng) and brachyury (ntl). eng could not be detected in the muscle pioneer cells of embryos injected with Xhox3. The pattern distribution of brachyury (Ntl) protein is abnormal in Xhox3 injected embryos. Evidence for the conservation of the NH2 terminal region of the Xhox3 protein in frogs and fish is provided by the detection of a nuclear Xhox3-like protein in 24 h zebrafish embryos located in posterior mesoderm tissue. Previous results in Xenopus embryo research and the data presented here support the conservation of an A-P patterning mechanism involving the Xhox3 gene.