We have discovered a new type of haemocyte in the larval stage of the tobacco hornworm moth Manduca sexta that has extreme phagocytic ability; each cell can engulf up to 500 bacteria. This level of phagocytosis may be unprecedented among animal cells. Although these hyperphagocytic cells (HP) only represent about 1% of the circulating haemocytes, they are responsible for sequestering the majority of the bacteria by circulating haemocytes when non-pathogenic, heat-killed Escherichia coli are injected into the haemolymph. Extreme phagocytosis by HP is not limited to Gram-negative bacteria since heat-killed Staphylococcus aureus as well as positively and negatively charged microspheres are also highly phagocytosed. Evidence is presented to show that phagocytosis by HP is involved in the early stages of nodule formation in infected insects. In addition, HP are also present in non-infected insects, characterised by their distinctive spreading morphology, which becomes impaired following hyperphagocytosis of bacteria. This is the first time that a dedicated "professional" phagocytic class of haemocyte has been reported for an invertebrate. The importance of these specialised cell types in the M. sexta immune response and their role in nodule formation is discussed.