The relative importance of chemical antifouling defences in natural systems may be overestimated by the apparent emphasis on the publication of positive results. Here, the antifouling activity of crude tissue extracts from a guild of temperate zone sessile invertebrates is tested in a laboratory bioassay. Twelve of the 20 extracts significantly impeded the successful metamorphosis of larvae of the co-occurring solitary ascidian, Herdmania momus, at extract concentrations ≤5% of those occurring naturally in the invertebrate tissue. Sponge extracts were the most successful at impeding metamorphosis; five of the six most potent extracts were derived from this phylum. Sponges also yielded a significant correlation between the antifouling activity of extracts and the degree to which their surfaces were fouled in the field, although the removal of an outlying data point changed the nature and significance of this relationship. Nevertheless, it would appear that sponges may rely more heavily on chemical antifoulants than other phyla. In contrast to the antifouling activity detected, significantly enhanced metamorphosis relative to controls was noted for low concentrations of extracts from six invertebrate species. Extracts from three of these species had significantly impeded metamorphosis at higher concentrations. When all species were considered, no evidence was found of a correlation between the potency of the antifouling defence and the degree to which the surfaces of organisms was fouled. Indeed, extracts from two colonial ascidians were only moderately effective at dissuading foulers, yet their surfaces were never fouled. These findings are consistent with the existence of a suite of defences to combat foulers. Hence the degree to which species are fouled may not be a good indicator of their potential chemical antifouling activity.