Settlement bioassays with larvae of the cheilostome bryozoan Bugula neritina were used to isolate and purify antifouling compounds from the ascidian Eudistoma olivaceum (Van Name). Three inhibitors of larval settlement, two toxic and one nontoxic, were investigated. The toxic compounds accounted for approximately 0.4% of the organic fraction (on a dry weight basis) and were identified as eudistomins g and h, two of a number of alkaloids possessed by E. olivaceum. The nontoxic inhibitor of settlement was not characterized. Eudistomins g and h were effective inhibitors of larval settlement at concentrations as low as 0.5% of that present in the living animal. © 1990 Plenum Publishing Corporation.