This study describes morphological changes during capsule formation of Trichinella spiralis with emphasis on nuclei within the capsule. The nuclei of the pre-existing muscle cell may become hypertrophied or degrade soon after the entrance of newborn larvae. Cytoplasm associated with these hypertrophied nuclei was basophilic in H&E staining and composed of mitochondria and rough endoplasmic reticulum. Satellite cells underwent cell division and joined to the nurse cell. These nuclei may also become hypertrophied or degrade. Cytoplasm associated with nuclei of satellite cell origin was eosinophilic in H&E staining and composed of mitochondria, polysomes, and smooth and rough endoplasmic reticulum. Eosinophilic cytoplasm (of satellite cell origin) became prevalent in the capsule as capsule formation proceeded. Infiltrated cells were observed within the capsule. Thus this study showed diversity of nuclei in the capsule of Trichinella.