AbstractInDrosophila melanogasterthere are two genes encoding ribosomal protein S5,RpS5aandRpS5b.Here, we demonstrate thatRpS5bis required for oogenesis. Females lackingRpS5bproduce ovaries with numerous developmental defects that undergo widespread apoptosis in mid-oogenesis. Females lacking germlineRpS5aare fully fertile, but germline expression of interfering RNA targeting germlineRpS5ain anRpS5bmutant background worsened theRpS5bphenotype and blocked oogenesis before egg chambers form. A broad spectrum of mRNAs co-purified in immunoprecipitations with RpS5a, while RpS5b-associated mRNAs were specifically enriched for GO terms related to mitochondrial electron transport and cellular metabolic processes. Consistent with this,RpS5bmitochondrial fractions are depleted for proteins linked to oxidative phosphorylation and mitochondrial respiration, andRpS5bmitochondria tended to form large clusters and had more heterogeneous morphology than those from controls. We conclude that RpS5b-containing ribosomes preferentially associate with particular mRNAs and serve an essential function in oogenesis.