Identification and manipulation of the germ line are important to the study of model organisms. Although zebrafish has recently emerged as a model for vertebrate development, the primordial germ cells (PGCs) in this organism have not been previously described. To identify a molecular marker for the zebrafish PGCs, we cloned the zebrafish homologue of the Drosophila vasa gene, which, in the fly, encodes a germ-cell-specific protein. Northern blotting revealed that zebrafish vasa homologue (vas) transcript is present in embryos just after fertilization, and hence it is probably maternally supplied. Using whole-mount in situ hybridization, we investigated the expression pattern of vas RNA in zebrafish embryos from the 1-cell stage to 10 days of development. Here we present evidence that vas RNA is a germ-cell-specific marker, allowing a description of the zebrafish PGCs for the first time. Furthermore, vas transcript was detected in a novel pattern, localized to the cleavage planes in 2- and 4-cell-stage embryos. During subsequent cleavages, the RNA is segregated as subcellular clumps to a small number of cells that may be the future germ cells. These results suggest new ways in which one might develop techniques for the genetic manipulation of zebrafish. Furthermore, they provide the basis for further studies on this novel RNA localization pattern and on germ-line development in general.