The envelope around larvae of Moniliformis dubius appears to protect the parasite against immune recognition and encapsulation by the insect host's haemocytes. The origin of this envelope has been the subject of controversy although most evidence suggests it is parasite-derived. If host-derived, the envelope would be expected to share surface properties with host tissue. Thus, experiments were undertaken, transplanting parasites and host tissue to other insects and using haemocytic encapsulation as an assay for immune recognition, in order to compare the response to host tissue and to the parasite's envelope. Parasites without their envelopes, and pieces of tissue (ventral nerve cord) from the experimental host (the locust Schistocerca gregaria) were recognized as foreign and encapsulated in the cockroach, Periplaneta americana. The majority of parasites with their envelopes were unencapsulated or only partially encapsulated on transfer to their normal host, P. americana, indicating that the envelope does not have surface similarity to locust tissue. Cockroach-derived parasites with or without envelopes were not encapsulated in S. gregaria, suggesting that the larva itself can evade or inhibit the locust's recognition mechanism. However, since larvae which develop in S. gregaria are enclosed in an envelope, the formation of the envelope would seem to be an inherent feature of the parasite's development.