Exposure of larvae of Galleria mellonella larvae to mild physical (i.e. shaking) or thermal stress for 24 h increased their ability to survive infection with Aspergillus fumigatus conidia however larvae stressed in a similar manner but incubated for 72 h prior to infection showed no elevation in their resistance to infection with A. fumigatus. Stressed larvae demonstrated an elevated haemocyte density 24 h after initiation of the stress event but this declined at 48 and 72 h. Larval proteins such as apolipophorin, arylophorin and prophenoloxidase demonstrated elevated expression at 24 h but not at 72 h. Larvae maintained at 37°C showed increased expression of a range of antimicrobial and immune-related proteins at 24 h but these decreased in expression thereafter. The results presented here indicate that G. mellonella larvae are capable of altering their immune response following exposure to mild thermal or physical stress to mount a response capable of counteracting microbial infection which reaches a peak 24 h after the initiation of the priming event and then declines by 72 h. A short-term immune priming effect may serve to prevent infection but maintaining an immune priming effect for longer periods may be metabolically costly and unnecessary while living within the colony of another insect.