A gene expressed in the choroidal fissure of the zebrafish eye was isolated. This gene, designated #61, contained significant homology with the previously reported actin-binding protein smoothelin. During zebrafish embryogenesis, #61 expression was first detected in the lateral mesoderm of the mid-trunk region, and then strong expression was observed in the choroid fissure of the eye and in a part of the brain at 30 hpf. Abrogation of #61 activity by an antisense morpholino oligonucleotide resulted in the failure of closure of the choroid fissure at 30 hpf. In addition, hemorrhage was observed at the caudal side of the eye. Detailed analysis indicated that leakage of blood may have arisen from the hyaloid vessels and the primordial midbrain channels. On the other hand, retinal differentiation and optic nerve formation seemed normal. Taken together, our data suggest that gene #61 may play a role in the formation of hyaloid vessels and subsequent choroid fissure closure. (C) 2003 Elsevier Inc. All rights reserved.