Centrin is a calcium-binding phosphoprotein of centrosomes, mitotic spindle poles, and flagellar basal apparatus. Indirect immunofluorescence studies in human and rat retinas reveal centrin localization in two distinct cellular structures: at centrosomes of nonciliated neuronal cells as well as in basal bodies, and in larger amounts in the highly modified cilium--the connecting cilium--of photoreceptor cells. Western blot analyses of mammalian retinal proteins show two closely migrating centrin bands at about 20 kDa, the previously described molecular weight of centrins. Using isoform specific primers in PCR, the expression of two related but distinct forms of centrin (centrin 1 and centrin 2), can be identified in the retina of human and rat as well as in the mammalian testis, tissues where cilia are present. However, only one isoform (centrin 2) is expressed in nondifferentiated, nonciliated retinal cells (retinoblastoma cells), as well as in rat liver, skeletal muscle, and cardiac muscle. These observations suggest centrin 2 message may be universally expressed while centrin 1 message may be restricted to retina and testis which contain cells that have differentiated cilia or flagella, or their modifications.