The KCNE family (KCNE1-5) is a group of single transmembrane auxiliary subunits for the voltage-gated K(+) channel KCNQ1. The KCNQ1-KCNE complexes are crucial for numerous physiological processes including ventricular repolarization and K(+) recycling in epithelial cells. We identified a new member of the KCNE family, "KCNE6", from zebrafish. We found that KCNE6 is expressed in the zebrafish heart and is involved in cardiac excitability. When co-expressed with KCNQ1, KCNE6 produces a slowly activating current like the slow delayed-rectifier K(+) current (I(Ks)) induced by KCNE1, despite the fact that the KCNE6 amino acid sequence has the highest similarity to that of KCNE3, which forms a constitutively open channel with KCNQ1. The kcne6 nucleotide sequences exist throughout vertebrates, including humans, although only the KCNE6 proteins of lower vertebrates, up to marsupials, can modulate KCNQ1, and it has become a pseudogene in eutherians. Our findings will facilitate a better understanding of how the KCNE family has evolved to modulate KCNQ1.