Abstract. We have generated transgenic zebrafish lines expressing a fusion of a histone variant, H2A.F/Z, to the green fluorescent protein (GFP) of the jellyfish Aequorea victoria. Here, we describe the molecular cloning, partial characterisation and expression of the zebrafish H2A.F/Z histone gene, as well as the construction of the transgene and its transformation into the zebrafish germ line. No abnormality can be detected in transgenic fish expressing the H2A.F/Z:GFP fusion protein. The nuclear localisation of the fusion protein correlates with the start of zygotic transcription, in that it is present in the unfertilised egg and in the cytoplasm of cells after the first cleavages, being found in some nuclei after the seventh or eighth cleavage, whereas all nuclei from the 1,000-cell stage on, i.e. after midblastula transition, contain protein. In addition to these data, we present a few examples of the many possible applications of this transgenic line for developmental studies in vivo. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00427-001-0196-x