The pea aphid, Acyrthosiphon pisum, can host different facultative symbionts (FS), which may provide various benefits to the host, including adaptation to the host plant and resistance to heat or natural enemies (fungi, bacteria, parasitoid wasps). Here, we searched whether and how the presence of some FS could affect a key component of insect innate immunity, the phenoloxidase, under normal and stressed conditions. For this, we used A. pisum clones of different genetic background (LL01, YR2 and T3-8V1) and harboring or not FS (Regiella insecticola (Ri), Hamiltonella defensa (Hd) or Serratia symbiotica (Ss)). Proteomic analysis of aphid hemolymph and PCR indicated that the two A. pisum phenoloxidases, PO1 and PO2, are expressed and translated into protein. They seem mainly secreted as circulating enzymes in the hemolymph and a proteolytic cleavage was not necessary for their activation. PO genes expression was dependent upon the aphid genotypes as well as the amount of PO proteins and activity in the total hemolymph (T3-8V1-Amp > LL01 = YR2-Amp). The presence in YR2 and T3-8V1 clones of Hd or Ri, but not Ss, caused a sharp decrease in PO activity by interfering with both transcription and translation. Microinjection of different types of stressors (yeast, E. coli, latex beads) in YR2 lines affected the survival rate of aphids and in most cases, it also decreases the PO genes expression after 24h, whereas the amount and activity of the proteins varied differently depending on the FS and the stressor, regardless of the genes expression. These data provide new hypothesis on the mechanism by which some facultative symbionts act on the pea aphid immunity.