The film is a documentation of the non-cellular defense reaction (humoral encapsulation) of larvae of Chironomus thummi (Insecta; Diptera) against parasitic nematodes. The result of this reaction is first demonstrated in vivo. A dead parasite (Mermithidae, Hydromermis contorta), surrounded by a brownish crust, is shown in the body cavity of a Chironomus larva. The formation of the capsule can better be observed in vitro in isolated hemolymph. The deposition of the material begins within 2-5 minutes after addition of the mermithid. The nematode, first swimming around actively, begins to have more and more difficulties mowing. It is obvious that the surface of the parasite becomes sticky. The animl adheres to the underground and finally is completely motionless. At high magnification the deposition of the capsule can be studied in detail. On the cuticle of the mermithid droplets appear, which increase in number and size until a complete envelope is formed. The trapped parasite can be seen inside the capsule; it attemps to move and sometimes escapes by breaking the capsule. In the final part of the film the example of Turbatrix aceti shows that humoral encapsulation of Chironomus hemolymph is also efficient against non parasitic nematodes which are encapsulated in the same manner as the parasitic mermithids.