The spontaneous mutant bu (bubble) of the multicellular green alga Ulva mutabilis develops in its most extreme form as a hollow sphere consisting of only one cell type, while the wild type has a differentiated thallus consisting of rhizoid cells, giant stem cells, and blade cells. In the early development of the wild type, the planes of division are perpendicular to the long axis of the plant. Later, the restrictions of the division planes are partly released and the planes are perpendicular to the surface of the plant and otherwise arbitrarily oriented. The mutant, throughout its development, has only the latter restrictions on its division planes. The early cell divisions of the mutant are synchronized in a diurnal light-dark cycle, in contrast to the early development of the wild type, but in agreement with what is found in its blade. In addition, newly differentiated blade cells, when isolated, develop into plants phenotypically similar to the mutant. These results suggest that the mutant cells are equivalent to the blade cells of wild type. The bu gametophytes derived from bu + bu sporophytes may develop either their own bubble phenotype, or phenotypes intermediate between bubble and wild type, or the wild-type phenotype. The frequency of the latter is lower by treatment of the spore mother cells with dactinomycin during a period immediately preceding meiosis and sporulation. It is also dependent on the length of the first period of light to which the zoospores are exposed. Accordingly, it is a premeiotic transcription of the bu + gene, and a product of the gene is transferred from the heterozygous ( bu + bu ) spore mother cells to the zoospores. When the zoospores are exposed to light for the first time, the bu + gene product becomes involved in processes which proceed during the light-dark sensitive period, and when completed, channel the development of the mutant progeny into the developmental path of the wild type. The predetermining effect of the bu + product is also observed among gametophytes derived from sporophytes heterozygous for bu and the semidominant mutant Sl (Slender). The presence of Sl in the heterozygous sporophyte has no effect on predetermination, even if Sl is phenotypically expressed in the heterozygote itself. But the presence of Sl in the zoospores reduces the expression of the predetermination, and this implies that the Sl + gene is of importance for the completion of the bu + processes of the zoospores. The blade cells form the reproductive units (zoospores and gametes) in Ulva, and the properties of the mutant can be explained if the bu + gene is concerned with the removal of restrictions imposed on the differentiated blade cells, so that the total information in the genome can again be used to direct a normal development.