Using genomic DNA from late-onset retinal degenerate and wild type Labrador Retrievers as templates and canine exon-specific oligonucleotides as primers in polymerase chain reaction, all four introns of opsin were cloned and sequenced. Dot-matrix comparisons were made for human, murine and canine introns. Selected sequences containing either intronic or coding sequences were aligned and used for phylogenetic relationship analysis. The opsin gene introns are conserved between the human, the mouse and the dog with regards to number and length. In addition there is an astonishingly high degree of sequence homology between the second and fourth introns. Introns 2(1277 bp in dog) and 4 (863 bp in dog) are 72% and 71% homologous to the human introns, and 57% and 52% homologous to the mouse introns, respectively. The coding sequence (CDS) of the dog shows 93% homology to human CDS and 88% homology to mouse CDS. A phylogenetic analysis of the intronic sequences 2 and 4 confirms the higher relatedness between dog and human than between mouse and human opsin genes. As there are good reasons to believe that the primate and rodent lineages are closer to each other than to the Canis familiaris, there must be some functional constraints on the evolution of human and dog opsins.