Members of the Rx (retinal homeobox) gene family play vital roles during eye development in vertebrates. In this thesis, a new Rx-type gene, XRxL, was identified from Xenopus. According to a phylogenic analysis, all-known Rx-type genes could be grouped into four categories, including the "invertebrate Rx" group, which contains all Rx genes from invertebrates, the "classical vertebrate Rx" group, the "vertebrate Rx-Q50" group, and the "vertebrate Rx-like" group to which XRxL belongs. The earliest expression of XRxL can be detected in the presumptive eye area at late neurula stage by WMISH. Suppression of XRxL function in vivo by microinjection of RxL-specific antisense morpholino oligonucleotides impaired the formation of the photoreceptor layer and reduced the expression of photoreceptor specific genes. Overexpression of XRxL induced ectopic expression of photoreceptor specific genes, but did not promote the proliferation of retinal progenitor cells significantly. Targeted overexpression of XRxL in developing retinoblasts in vivo led to the increased fraction of photoreceptor cells at the expense of amacrine and bipolar cells. Moreover, XRxL was found to promote both rod and cone photoreceptors, with a preference for rods. Our in vivo experiments also revealed that XRxL acts as a transcription activator. Taken together, XRxL, unlike XRx1, is required for the determination of retinal cell types, especially photoreceptor cells, rather than to promote the proliferation of retinal progenitor cells.