Sphaerospores were found in the kidneys of alevin channel catfish (Ictalurus punctatus) from a farm in Central California. Multicellular developmental stages, similar to C-blood protozoans described for Sphaerospora spp. from cyprinid fishes, were observed in circulating blood and numerous tissues. Upon a 2nd examination of the same population of fish 10 days later, sporogonic stages were seen developing into mature sphaerospores in the lumina of the kidney tubules. Sporogenesis was asynchronous with simple unicellular stages adjacent to more complex forms with developing polar capsules and valves. Only one elliptical spore (5.6 microns in width, 6.5 microns in thickness by 5.8 microns in length) developed within the surrounding pseudoplasmodium. Thin valves surrounded two sporoplasm cells and two subspherical polar capsules (1.7 x 1.9 microns) which contained a polar filament with four to five turns. The blood stages of the Sphaerospora sp. described here are similar to the trophozoites seen in channel catfish with proliferative gill disease (PGD). Early stages of PGD also observed in the same population of channel catfish containing developmental and sporogonic stages of this newly recognized Sphaerospora sp. may suggest a causal relationship between this new myxosporean and the gill disease.