Our objective is to describe in detail the way in which the patterns of protein synthesis are modified by the action of specific control genes during development of the simple eukaryote Volvox carteri. The control gene mutations to be analyzed include those at the reg A locus (which cause somatic cells to dedifferentiate and redifferentiate as reproductive cells), the lag locus (which cause delayed development of reproductive cells) and the three sex c loci (which cause development of sexual individuals in the absence of the sexual inducer normally required). In vivo protein synthetic patterns of individual cell types throughout asexual and sexual development of wild type and mutant Volvox are analyzed by SDS-PAGE of polypeptides synthesized during a one hour labelling period. Because substantial control of protein synthesis has already been shown to be exerted at a posttranscriptional level, comparisons will be made between in vivo labelling patterns and those obtained by translation of isolated mRNAs in cell free protein synthetic systems.