The zebrafish, Danio rerio, is becoming an increasingly popular model for the study of vertebrate development. Indeed, the biology of the fish offers great advantages for such studies. The life cycle of the zebrafish is relatively short (2-3 months) and the embryos develop outside the mother, facilitating the visualization of any mutated phenotype. At present, more than 1000 embryonic mutations have been reported. However, until recently, there was no physical or genetic map for this organism. In an effort to generate such a map, we have produced and characterized a panel of zebrafish-mouse cell hybrids. We have used whole-cell fusion to transfer zebrafish chromosomes from two different zebrafish cell lines into mouse recipient cells, thus generating more than 100 hybrids. Using fluorescence in situ hybridization and polymerase chain reaction analysis, we have determined the zebrafish chromosome composition of these hybrids. Here we report that elements from the 25 linkage groups of the zebrafish genome are present in our hybrids. These hybrids could identify the chromosomal location of genes affected in zebrafish mutants.