The establishment of polarity axes in the Drosophila egg and embryo depends upon the localization and on-site expression of maternal mRNAs. The critical step in the targeting of posterior determinants is the localization of oskar (osk) mRNA to the pole and its on-site translation. Osk protein then recruits other posterior group gene products involved in the formation of pole plasm and in the localization and regulation of the posterior determinant, nanos. Here we have investigated the role of the Drosophila CPEB homolog, the orb gene, in the osk mRNA localization pathway. We demonstrate that the expression of Osk protein is dependent upon the orb gene. In strong orb mutants, Osk protein expression is undetectable, while in the hypomorphic mutant, orb(mel), little or no on-site expression of Osk protein at the posterior pole is observed. The defects in Osk protein accumulation in orb mutant ovaries are correlated with a reduction in the length of the osk poly(A) tails. We show that osk mRNA is in immunoprecipitable complexes with Orb protein in ovaries and that the osk 3' UTR can be UV cross-linked to Orb protein in ovarian extracts. These data suggest that Orb is required to activate the translation of osk mRNA and at that this may be accomplished by a mechanism similar to that used by the Xenopus CPEB protein to control translation of "masked" mRNAs.