Almost nothing is known about the physiology of the pronephros other than that it is efficient at excreting large volumes of dilute urine. The distal segment of pronephric nephrons is the most poorly characterized region of this organ, in part due to the absence of early markers of this structure. In this report the embryonic expression of a Cl(-) conductance channel, x ClC-K, is described with high level transcription of this gene in the distal segment beginning at Xenopus stage 31. x ClC-K is also expressed at lower levels in the pronephric duct. This gene serves as a convenient marker for investigating the development of this physiologically critical nephric segment and also serves to identify the potential site of action of a component of the chloride handling system of the pronephros. As ClC-K channels are involved in human kidney disease the frog pronephros may be a useful system in which to study the in vivo activity of this channel.