PURPOSE. To investigate the morphology and function of photoreceptors in mice with mutation of the FSCN2 gene. METHODS. A mouse line was generated carrying the 208delG mutation (point mutation, or p-type) and another with replacement of exon 1 by the cDNA of a green fluorescent protein (GFP knock-in, or g-type). The expression of retinal mRNA was determined by reverse transcription (RT)-polymerase chain reaction (PCR) and in situ hybridization performed on retinal sections. Morphologic analyses of the retinas were performed by light microscopy (LM) and transmission electron microscopy (TEM) and functional analyses by electroretinogram (ERG). RESULTS. mRNA of FSCN2 was not detected in the retinal mRNA extracted from FSCN2p/p and FSCNg/g mice. Both FSCN2+/P and FSCN2+/g mice had progressive photoreceptor degeneration with increasing age detected by LM and structural abnormalities of the outer segment (OS) detected by TEM. Both FSCN2+/p and FSCN2+/g mice had depressed rod and cone ERGs that worsened with increasing age. CONCLUSIONS. These results indicate that haploinsufficiency of the FSCN2 gene may hamper maintenance and/or elongation of the OS disks and result in photoreceptor degeneration, as in human autosomal dominant retinitis pigmentosa.