When 10 μl of silicone oil was injected into the hemocoel of a 5-day-old 5th-instar larva of Samia cynthia ricini, the globular oil was encapsulated by hemocytes. The initial process of encapsulation was examined with scanning electron microscopy (SEM). Within 2 min after injection, granulocytes attached to the oil surface, and a thin membrane of amorphous substance occurred around the granulocytes. The cells immediately discharged numerous granules. A reticular network appeared around the degranulated cells, resulting in the formation of the innermost layer of the developing capsule. Compaction of capsule-forming hemocytes occurred after completion of the innermost layer of the developing capsule. These results suggest that a localized coagulation of hemolymph on the oil surface, which resembles ordinary hemolymph coagulation, constitutes the initial phase of encapsulation.