The majority of mitochondrial phosphatidylethanolamine (PtdEtn), a phospholipid essential for aerobic growth of yeast cells, is synthesized by phosphatidylserine decarboxylase 1 (Psd1p) in the inner mitochondrial membrane (IMM). To identify components that become essential when the level of mitochondrial PtdEtn is decreased, we screened for mutants that are synthetically lethal with a temperature-sensitive (ts) allele of PSD1. This screen unveiled mutations in PHB1 and PHB2encoding the two subunits of the prohibitin complex, which is located to the IMM and required for the stability of mitochondrially encoded proteins. Deletion of PHB1 and PHB2resulted in an increase of mitochondrial PtdEtn at 30°C. On glucose media, phb1Δ psd1Δ and phb2Δ psd1Δ double mutants were rescued only for a limited number of generations by exogenous ethanolamine, indicating that a decrease of the PtdEtn level is detrimental for prohibitin mutants. Similar to phb mutants, deletion of PSD1destabilizes polypeptides encoded by the mitochondrial genome. In aphb1Δ phb2Δ psd1tsstrain the destabilizing effect is dramatically enhanced. In addition, the mitochondrial genome is lost in this triple mutant, and nuclear-encoded proteins of the IMM are assembled at a very low rate. At the nonpermissive temperature mitochondria of phb1Δ phb2Δ psd1tswere fragmented and aggregated. In conclusion, destabilizing effects triggered by low levels of mitochondrial PtdEtn seem to account for synthetic lethality ofpsd1Δ with phb mutants.