PARASITIC infections are often associated with eosinophilia and high levels of immunoglobulin E (IgE). This observation has led to speculation that eosinophils and IgE may act together in the immune response against parasites. In support of this hypothesis, IgE and eosinophils participate in cytotoxic reactions directed against Schistosoma mansoni larvae in vitro^1,2. Furthermore, epidemiological studies have shown an inverse correlation between levels of specific IgE and rates of infection with Schistosoma^3–5. The low-affinity IgE receptor (FcεRII/CD23) was first incriminated in eosinophil activation^6,7. The fact that the high-affinity IgE receptor (FcεRI)^8,9 is not only expressed on mast cells and basophils but also on Langerhans cells^10,11 led us to investigate the presence of FcεRI on eosinophils. Here we show that FcεRI is expressed on eosinophils from hypereosinophilic patients, is involved in eosinophil degranulation, and participates in eosinophil-mediated cytotoxicity against S. mansoni. Our results indicate that FcεRI may play a major part in immune defence against parasites.