The males of many insect species transfer a spermatophore, i.e., a proteinaceous capsule containing sperm, to females during copulation, and this may also function as a nuptial gift. If production of the spermatophore is costly and variations in the quality of females are large, males may strategically allocate their investment based on the quality of the mate to maximize their own reproductive success. We examined the size and protein content of spermatophores transferred to females of different ages and body sizes, and also to water-deprived and water-replete females in the moth Ostrinia scapulalis (Walker). Males transferred a spermatophore of a smaller size or with less protein to older females, smaller females, and water-deprived females. These results indicated that O. scapulalis males manipulated their reproductive investment based on the conditions of the mate. We also demonstrated that older males varied their resource allocation to a greater extent in response to female conditions than younger males. Thus, resource allocation by the males of this species is modulated by both female conditions and the age of the males.