For the first time, a functional study of haemocytes from the crab Carcinus aestuarii was performed inorder to evaluate their involvement in immune responses. Total haemocyte count (THC), phagocytosis,haemolymph opsonisation properties, hydrolytic and oxidative enzyme activities, and production ofintracellular superoxide anion were evaluated. A great variability in THC was recorded among individuals,and haemocyte mean number was 6.4 (×10⁶) cells/ml haemolymph. Although only hyalinocytes were able to phagocytose yeast cells or Zymosan, phagocytic index was low (3%) and did not increase significantly (4%) after pre-incubation of yeast and Zymosan in cell-free haemolymph, suggesting that haemolymph did not have opsonising properties. All haemocyte types produced superoxide anion,whereas only granulocytes were positive to the hydrolytic enzymes assayed. In addition, only granulocytes were positive to phenoloxidase activity. Both Petri dish and spectrophotometric assays revealed a very low lysozyme-like activity in cell-free haemolymph (CFH) and haemocyte lysate (HL), although enzyme activity was higher in CFH than in HL. Interestingly, normalisation of data as to total protein content in CFH and HL resulted in an opposite situation, lysozyme-like activity being higher in HL than in CFH. This demonstrated that haemolymph of C. aestuarii has a high quantity of total proteins, functional properties of which need to be better investigated in future studies. Overall, the results obtained in the present study indicated that C. aestuarii haemocytes are not very active phagocytic cells, but they are more active in terms of both hydrolytic and oxidative enzyme activities and superoxide anion production. Fish & Shellfish Immunology 28 (2010) 534-541，Valerio Matozzo, Maria Gabriella Marin，Department of Biology, University of Padova, Via Ugo Bassi 58/B, 35131 Padova, Italy.