Isolation of polyacetylenes from aerial and subterranean parts of water hemlock, Cicuta virosa L. (Apiaceae. Apioideae), yielded eleven C (17)-polyacetylenes: cicutoxin, cicutol, (8 E,10 E)-heptadecadiene-4,6-diyne-1,12-diol (cicudiol, 7), 2,3-dihydrooenanthetol and (8 E)-heptadecene-4,6-diyne-1,10-diol already known to occur in water hemlock, the configurational isomers of cicutoxin (isocicutoxin) and of cicutol (isocicutol, and two incompletely characterised isomers with two cis-double bonds), all of them not described in previous investigations, falcarindiol, (1,8 E,10 E)-heptadecatriene-4,6-diyn-3-ol, already known from other Apiaceae, and the novel polyacetylene (1,8 E/ Z,10 E,12 E)-heptadecatetraene-4,6-diyn-3-ol. Keto compounds, postulated to occur in water hemlock, could not be detected. HPLC profiles of the lipophilic extracts of different organs, individuals, and provenances of water helmock harvested at different seasons revealed quantitative but not qualitative variations of the polyacetylene composition.