The mammalian voltage-gated potassium channel EAG1 is predominantly expressed in the retina and in the brain. The function of EAG1 channels is not understood in detail, but the strong expression in neuronal tissues suggests a role in electrical signaling. Ion currents conducted by EAG1 channels were extensively investigated in heterologous expression systems and in cancer cell lines, where EAG1 channels apparently promote cell proliferation. Nevertheless, the physiological functions of EAG1-mediated currents still remain elusive and further research requires a suitable model organism, which exhibits endogenous EAG1 expression and allows a faithful comparison with the human isoform. The zebrafish (Danio rerio) has been widely used as a vertebrate model system. Therefore, we surveyed the genomic databases of Danio rerio and found several potential EAG genes that were predicted using in-silico methods. Two paralogous genes exhibit homology to EAG1 and are located on chromosomes 17 (zEAG1A) and 22 (zEAG1B). The identity between the corresponding amino acid sequences of Danio rerio and the human EAG1 is more than 80%. We verified and characterized the endogenous expression of the EAG1 genes in Danio rerio by analysis of mRNA from different tissues and at various developmental stages, followed by reverse transcription and identification of EAG-gene specific sequences by PCR. Full-length expression clones were prepared and functionally analyzed upon expression in Xenopus oocytes. Using the two-electrode voltage-clamp method, we showed that the gene products form functional potassium channels exhibiting EAG-specific characteristics known from other species, a strong dependence of activation kinetics on prepulse potential and extracellular Mg²⁺ concentration. In addition, we found a splice variant of the zEAG1A gene on chromosome 17 that codes for a potassium channel with an extended S3-S4 linker. While a similar variant of bovine EAG1 (bEAG1b) only weakly differs from the short bEAG1a isoform with respect to its Mg²⁺ dependence (Frings et al., 1998, JGP 111:583), the alternatively spliced isoform in Danio rerio (zEAG1Ab) exhibits a substantially faster activation gating compared with zEAG1Aa. Rise times to 80% of the maximum current elicited by depolarizations to +20 mV from a holding of –120 mV in the presence of 5 mM external Mg²⁺ were 460  8 ms and 44  2 ms (n=22) for zEAG1Aa and zEAG1Ab, respectively. This suggests a regulatory role of the alternatively spliced S3-S4 linker for channel function. Given this marked phenotype, the Danio rerio channels provide a suited system for unraveling the underlying molecular mechanisms. The endogenous expression of EAG genes in Danio rerio and the similarity of the resulting currents in comparison to those carried by human EAG channels render the zebrafish a promising model for studying the physiological roles of EAG potassium channels and their potential involvement in developmental processes.