The present study investigated aspects of the antifoulant properties of three sympatric species of ascidians found in seagrass habitats of the Gulf of Mexico, Southern Atlantic Ocean, and Caribbean. Field observations in Saint Joseph Bay, Florida indicate that all three species are common and that the tunic of the solitary ascidian Molgula occidentalis is often heavily fouled, while the outer surfaces of both the colonial ascidians Amaroucium stellatum and Botryllus planus are free of fouling organisms. Antifoulant activities of a suite of increasing hydrophilic organic extracts prepared from the tunic of M. occidentalis and whole colonies of A. stellatum and B. planus were measured using both sympatric microbial (bacteria) and macroinvertebrate (cyprid larvae of Balanus amphitrite) fouling organisms in laboratory bioas-says. In addition, field antifoulant assays were conducted by combining organic extracts with controlled-release resin and subsequently coating this material on to acrylic rods deployed in the field for a 72 h period. Extracts of the tunic of M. occidentalis generally did not inhibit bacterial growth. The exception was the methanol extract, which inhibited growth in one of the six marine bacteria tested. Moreover, only the highest concentrations of hexane and methanol tunic extracts tested prevented attachment of cyprid larvae. Field assays revealed no antifoulant activity on rods coated with resin containing extracts of M. occidentalis. Inhibition of both microbial growth and cyprid settlement were much more pronounced in whole-organism extracts of the two colonial ascidians. Most potent were the aqueous methanol extracts of colonies of B. planus and A. stellatum which inhibited growth in five of the six marine bacteria tested. In addition, hydrophilic and lipophilic extracts of the colonial ascidians significantly inhibited attachment of cyprid larvae, in many instances across a wide range of extract concentrations. Field antifoulant assays indicated that extracts of both colonial ascidians inhibited settlement of bryozoans and barnacles. The findings indicate that the colonial ascidians B. planus and A. stellatum possess chemical antifoulant properties. In contrast, the solitary ascidian M. occidentalis appears to either tolerate fouling or possess other non-chemical mechanisms to cope with the risks associated with epibiont overgrowth.