A full-length cDNA for retinoblastoma (RB1) has been cloned from a cDNA library prepared from 3-week-old rainbow trout (Oncorhynchus mykiss) eyed embryos. The trout RB1 cDNA encodes a predicted protein of 910 amino acids and is the most divergent cloned retinoblastoma gene sequence to date. RT-PCR studies reveal high levels of RB1 expression by the second week of embryogenesis, which remains uniformly expressed until hatching. Expression studies of adult fish tissues show the RB1 gene to be expressed in all tissues examined, including the oesophagus, eye, liver, intestine, posterior and anterior kidney, skin, stomach, muscle, spleen, gill, swim bladder, gonads and brain. The RB1 gene appears to be a single copy gene based on Southern analysis, and maps to linkage group XVI in the trout genome map. Polymorphisms in the RB1 gene and in closely linked markers should facilitate LOH analysis of RB1.