Much of our current understanding of how cytoplasmic determinants are localized and activated stems from genetic analyses in Drosophila. Characterization of mutants defective in establishing the initial embryonic body pattern has revealed the importance of mRNA localization and translational control and identified several factors required for these processes. Nevertheless, many additional genes are likely to be involved. Here, we describe a novel genetic screen designed to identify genes that participate in posterior body patterning, an elaborate process involving the sequential use of two localized cytoplasmic determinants, the products of the oskar and nanos genes. From the screen, we recovered new alleles of genes known to be required for posterior body patterning, demonstrating the validity of the approach. In addition, we isolated numerous other mutants. Further characterization of one mutant, P58, revealed that it is a novel allele of bullwinkle. We find that in bullwinkle mutants, oskar mRNA localization is not maintained in the embryo and oskar protein accumulates ectopically and to abnormally high levels. These defects are distinct from previously described perturbations in oskar activity and provide new insights into the regulation of oskar.