Bioassays were conducted to determine the susceptibility of egg masses and young larvae of two pine processionary moth species, Thaumetopoea pityocampa and Thaumetopoea wilkinsoni, to two strains (ARSEF4556, V275) of the entomopathogenic fungus Metarhizium brunneum. Mortality of treated eggs by both strains ranged from 96% to 99% but not all of this was caused by M.brunneum since control groups also experienced egg mortality due to saprophytic fungi. Still, larvae hatched in the laboratory from eggs treated with M.brunneum were all killed by this fungus, acquiring M.brunneum conidia, whereas larval mortality was 0% in the control groups. Young larvae of both pine processionary moth species were also highly susceptible to ARSEF4556 and V275 with larval mortality ranging between 94% and 100%, 8 days post-inoculation, with the vast majority of larvae being killed within the first 2-4 days. Larval mortality was dose dependent. Results were consistent across the two pine processionary moth species, showing that the pathogenicity of M.brunneum to both eggs and young larvae might be promising for biological control of these insect pests. The study also showed that non-target parasitoids of pine processionary moth eggs were also susceptible to M.brunneum. Further work is required to understand and reduce the M.brunneum effect on non-target insects.