The protein composition of the cestode Schistocephalus solidus was measured in an experiment simulating the transition of the parasite from a cold-blooded to a warm-blooded host. Infective S. solidus plerocercoids obtained from the three-spined stickleback Gasterosteus aculeatus were heated at 40 °C for 1 hour or cultured in vitro at 40 °C and 22 °C for 48 hours. Plerocercoids taken out from the body cavity of sticklebacks immediately after their sacrifice were used as a reference group.