The vanadocytes of living juvenilePhallusia mammillataascidians were examined by utilising cytochemical localisation of vanadium(III)in situ, to help determine the commencement of vanadium assimilation. The first visual indication of accumulation of vanadium(III) within the vanadocytes occurs at five days after fertilisation, shortly after the siphons of the developing larvae have opened. At nine days a number of different cell types, including vacuolated amoebocytes, a cell type which possess numerous compartments, together with bivacuolated cells and signet ring cells, all stain positively for vanadium(III) with the ligand 2.2′-bipyridine.