Monoclonal antibodies (Mabs) specifically against haemocytes of shrimp (Fenneropenaeus chinensis) were employed to study the ontogenesis of haemocytes by immunofluorescence assay technique (IFAT) with the aids of avidin-biotin complex method. Different developmental stages were identified by means of histochemical staining. Stages including blastula, gastrula, limb bud stage, embryonic-nauplius and nauplius were examined. With Mab 2A3 against both hyline, semi-granular and granular haemocytes, the first positive cells were detected at the limb bud stage of the embryos. Distinguishable positive haemocytes with a diameter of about 7 microm appeared in embryonic-nauplius. Whereas, with Mab 1H11 specific to cytoplasmic granulars in haemocytes, the first positive granular haemocytes were observed at the stage of embryonic-nauplius. Increased number of both total and granular haemocytes at the stage of nauplius impressed a possible enhancement of the immune system as the embryogenesis progressed.