After the irradiation treatment with doses ranging from 0.1 to 0.5 kGy, the melanization process was significantly inhibited in the 1st and 2nd instar larvae of the khapra beetle, Trogoderma granarium Ev., killed by freezing on the 1st and 2nd week after irradiation. Also, irradiation inhibited significantly the melanization of the 4th instar larvae after their death, but the relationships between the dose, time elapsed after irradiation and the degree of melanization were not clear. Double freezing and tawing of control larvae increased the melanization index from 26.4 to 52%. Multiple freezing of irradiated larvae had no effect on further increase of melanin formation. Phenyloxidase activity was very low in irradiated larvae, and only single larvae produced red/black color with 2-methyl-DOPA the spot. The effect of time elapsed after the irradiation was evident. Almost all larvae irradiated with a 0.3 or 0.5 kGy dose failed to produce the color with the substrate, when assessed at the 2nd and 3rd week after the irradiation. However, the changes in melanization of the khapra beetle larvae cannot be used for indicating previous exposure of these insects to irradiation, because of the great variability in response of the melanization process to the irradiation treatment.