IN the course of experiments on Ulva mutabilis1 some gametophytes are occasionally found forming swarmer which copulate neither with +-gametes nor with −-gametes. These swarmers are diploid, as revealed by cytological and genetical examination. They are biflagellate and of normal size, and otherwise they fully resemble normal gametes. As do normal gametes, they also remain mobile for two days; then they fix themselves to the glass and develop into zoospore-forming plants. From the zoospores normal + and − gametophytes arise in equal numbers.