Members of the YidC/Oxa1/Alb3 protein family function in the biogenesis of membrane proteins in bacteria, mitochondria and chloroplasts. In Escherichia coli, YidC plays a key role in the integration and assembly of many inner membrane proteins. Interestingly, YidC functions both in concert with the Sec-translocon and as a separate insertase independent of the translocon. Mitochondria of higher eukaryotes contain two distant homologues of YidC: Oxa1 and Cox18/Oxa2. Oxa1 is required for the insertion of membrane proteins into the mitochondrial inner membrane. Cox18/Oxa2 plays a poorly defined role in the biogenesis of the cytochrome c oxidase complex. Employing a genetic complementation approach by expressing the conserved region of yeast Cox18 in E. coli, we show here that Cox18 is able to complement the essential Sec-independent function of YidC. This identifies Cox18 as a bona fide member of the YidC/Oxa1/Alb3 family. © 2007 The Authors.