CHEMOTAXIS of animal sperm, long thought not to occur1–4, was first proved in the marine coelenterate Campanularia5 and since then has been observed in other hydroids6,8. The species-specificities and cross reactions between sperm and reproductive structures of these species and genera have been described6, and some of the attractants have been isolated and their chemical properties reported7. In spite of circumstantial evidence for sperm chemotaxis in other phyla13,14 the sperm behaviour leading to aggregation has not been described. I now report evidence of sperm chemotaxis to egg water and egg extracts (see Table 1 for details) of Ciona intestinalis (Protochordata: Urochordata), a primitive chordate. These results imply that sperm chemotaxis either has evolved independently in groups widely separate on the phylogenetic scale or is widespread in the animal kingdom.