The ethanolic body-wall extracts of 16 species of echinoderms from 16 genera were screened for their ability to affect the attachment of the marine bacteria Deleya marina (Baumann) and Alteromonas luteo-violacea (Gauthier). Body-wall extracts were tested at concentrations which mimic mean natural tissue concentration, 3.0 mg/ml seawater, and four half-log dilutions of this initial concentration. The extracts of three echinoderm species caused significant inhibition of bacterial attachment, while extracts of eight species caused significant enhancement of attachment. The body-wall extract of the asteroid Goniaster tesselatus (Lamarck) displayed the most potent antimicrobial activity, completely inhibiting attachment of both bacterial species at a concentration of 3.0 mg/ml seawater. The ethanolic extracts of 20 echinoderm species were also tested at a similar range of concentrations for their ability to affect the settlement of cyprid larvae of the barnacle Balanus amphitrite (Darwin), and coronate larvae of the bryozoan Bugula neritina (Linne). All echinoderm extracts inhibited settlement of both barnacle and bryozoan larvae at the highest concentration tested (3.0 mg/ml seawater). Eleven of the 20 echinoderms tested (13 asteroids, 3 holothuroids, 3 ophiuroids and a crinoid) had body-wall extracts that inhibited settlement of competent barnacle and bryozoan larvae at concentrations as low as 0.12 mg/ml seawater. These extracted echinoderm compounds may function as non- toxic or toxic antifoulants, and to promote bacterial surface colonization, which could be valuable to the organisms disease resistance.