To begin to examine the function of genes that control early development in the hindbraln, we have screened an embryonic zebrafish cDNA library with a murine krox-20 gene probe that contained the conserved zinc finger regions. We have isolated two overlapping cDNAs, zf187 and zf201 which are homologues of the murlne krox-20 gene. The N-termlnal of the longest cDNA (zf201) contains two acidic regions identical to those of the murlne krox-20. This indicates that the functional organisation of these proteins is probably conserved. Northern Blot analysis identified a single transcript of 2.0 kb. Wholemount in situ hybridisation established that expression of the zebraflsh gene (krx-20) first appears at 100% eplboly as a single anterior domain of the prospective neuroeplthelium, followed very soon after by a second more posterior domain. The alternating pattern of expression of this gene In rhombomeres(r) r3 and r5 is apparent by 12 hr post-fertilisation, that Is prior to the morphological appearance of the rhombomeres. Around 14 hr neural crest migration begins from the dorsal surface of r5, moving caudally Into r6 and then ventrally towards the pharyngeal arches. Crest migration is not apparent at or after 16 hr. No neural crest migration was observed from r3. Expression of krx-20 is down regulated firstly in r3 around 26 hr and later in r5 around 30 hr.