Invertebrates are exposed to various pathogenic microorganisms in a similar way to vertebrates. The basic strategies of the self defense mechanism of invertebrates are an innate immunity which includes phagocytosis or encapsulation and rejection. Cooper et al. showed the natural killer activity of small granular cells from earthworm against several human and mouse tumor cell lines. In this study, we examined the presence of perforin in the earthworm coelomocyte. Immunohistochemical analysis showed that only particular small granular cells and not large granular cells contained perforin granules. We also analyzed the presence of perforin mRNA expression by cross hybridization technique using a mouse probe. Sequence analysis showed that earthworm perforin contained a cystein-rich domain like mouse perforin and C9.