The Southern Indian Ocean marine worm Cephalodiscus gilchristi has been found to yield new cephalostatins 7-9. Tube-inhabiting marine animals of the genus Cephalodiscus (one of two divisions in the class Pterobranchia, Hemichordata Phylum) are rarely encountered. Only some 18 species are presently known and confined primarily to Antarctica. One Southern Hemisphere temperate region species Cephalodiscus gilchristi was recorded off the coast of South Africa in 1906 and described in more detail in 1915-17. In 1988 we summarized results from the first chemical study of this genus and isolation of the powerful (P388 ED50 10-7-10-9 pg/mL) cell growth inhibitor cephalostatin 1 (1) from C. gilchristi. Subsequently we described cephalostatins 2-4 and 5 where introduction of an aromatic C'-ring (cf. 2 corresponding to cephalostatin 6) was found to greatly reduce (P388 ED50 N pg/mL) the cytostatic activity. We now report that further detailed investigation of C. gilchristi antineoplastic constituents has led to the discovery of three new and remarkable cephalostatins designated 7-9 that exhibit potent growth inhibitory and cytotoxic activity against diverse human solid tumor types in the US. National Cancer Institute's new in vitro, disease-oriented antitumor screen; Equilibrium constants in THF for dimerization of several cesium enolates of benzylic ketones show a decrease as the ion-pair acidities of the parent ketones increase when steric factors are comparable.