The process of autotomy-induced cheliped regeneration in the megalopa of the mud crab,Rhithropanopeus harrisiihas been studied by light microscopy and transmission electron microscopy. Following blastema eruption, four time intervals were selected arbitrarily to cover the entire period of limb regeneration. Epidermal proliferation preceded muscle and nerve regeneration. On the basis of morphologic observations, nerve tissue regenerated from existing pedal nerve. It is proposed that blood vessels arose from epidermal differentiation, while muscle differentiated from immigrant cells. Three types of haemocyte, resembling those described in adult crustaceans, were identified in the haemolymph of the larval limb bud: granular haemocytes, hyaline cells and lipoprotein cells. Two other immigrant cell types, which appear to be primitive in nature and may be variants of a single cell type, are also described. Morphological evidence is provided to suggest that the more electron dense variant differentiated into muscle; this cell type is thought to be of mesenchymal origin. This contribution provides baseline information against which the effects of xenobiotics on cheliped regeneration can be assessed.