Methanol-water (4:1, v/v) crude extracts (50 mg mL−1) of 25 Jamaican medicinal plants were screened in vitro for anthelmintic activity using infective third-stage larvae of Strongyloides stercoralis. The most effective extract was further chemically scrutinized to isolate and identify the source of the bioactivity, and the efficacy of this compound was compared with ivermectin. Eosin exclusion (0·1 mg mL−1) served as the indicator of mortality in all bioassays. A crude extract of Eryngium foetidum (Apiaceae) was significantly (Probit Analysis, P<0·05) more potent than the other plant extracts, taking 18·9 h to kill 50% (LT50) of the larvae. Further, the petrol extract of E. foetidum was significantly more effective (Probit Analysis, P<0·05) at killing the larvae (LT50, 4·7 h) than either its methanol–water or dichloromethane extract. The latter two effected less than 1% larval mortality after 120 h. With bioassay-driven column chromatography of the petrol extract, trans-2-dodecenal (eryngial) was identified and chemically isolated as the main anthelmintic compound in E. foetidum. There was a significant difference between the 24 h LD50 values (mm) of trans-2-dodecenal (0·461) and ivermectin (2·251) but there was none between the 48 h LD50 values (mm): trans-2-dodecenal (0·411) and ivermectin (0·499) in vitro.