A study subjecting birds to different radio-frequencies and intensities revealed features of the receptor DCMU molecule that would be met by cryptochrome. Cryptochromes have been reported in the retina of several bird species, among them domestic chickens and migratory passerines. Two forms of cryptochrome 1, Cry1a and Cry1b, which are splice products of the same gene, and cryptochrome 2 were identified. Yet presence in the retina is only one prerequisite for cryptochrome to serve as receptor molecule for magnetic compass information; the other requirements mentioned above must also be met. It is therefore important to determine which of the cryptochromes forms the crucial radical pairs and where within the eye and also where inside the respective cells this cryptochrome is located. Cryptochrome 2 seemed a less plausible candidate, because its sequence contains a nuclear localization signal, which is not characteristic for a receptor molecule. The two forms of cryptochrome 1, in contrast, are cytosolic. They are not transmembrane proteins, but they could be fixed to a membrane or to the cytoskeleton to keep them in a specific alignment. We therefore focused our search on the two forms of cryptochrome 1. In the present paper, we report that Cry1a is located in the retina in a way that is in accordance with a function as magnetoreceptor molecule. Our study involves two bird species, domestic chickens and European robins. The two are not closely related, show marked differences in behavior and habitat, but have the same type of magnetic compass mechanism. Quantitative comparison of the immunolabeling intensity of retinae sampled at different times of day and at different seasons did not indicate any differences between these samples. Furthermore, there were no obvious difference between the right and the left eye. This was true for chickens and robins alike. Hence we here show only one representative example of each of the data sets. The results in both Undecylprodigiosin hydrochloride species were the same: Cry1a was found in one specific population of very slender photoreceptors of the single cone type. Double labeling with the Cry1a antiserum and the UV/V opsin antiserum showed that this receptor is the UV/V cone, which has a higher population density in robins than in chickens.