Two naturally-occurring polyacetylene derivatives are found inTagetes minutaL. andBidens bipinnataL. which, when released into the soil, may affect crop growth. They are alpha- terthienyl in roots ofT. minutaand phenylheptatriyne in leaves ofB. bipinnata.Soil samples were collected from a lot where pure stands of the two weeds were found to be growing. Similar soil was also sampled for control purposes, where the two weeds did not occur. The effect of aqueous extracts, taken from the weed-infested soils, on seed germination of seven horticultural crop species was determined in the laboratory. Nine crop plant species were also grown in pots containing the sampled soil. The species were: carrot, cucumber, lettuce, maize, onion, radish, squash, sunflower and tomato. Seed germination was delayed by aqueous extracts from the weed- infested soils. Where soil was infested withTagetesduring the previous season, dry mass of top growth and plant height of all the test plants were greatly reduced. Soil from an earlierTagetesinfestation had less of an effect on growth, with carrot and maize growing normally and growth of cucumber stimulated. Soil from aBidensinfestation significantly affected the eight plant species tested, although not to such a marked degree as soil from a recentTagetesinfestation.S. Afr. J. Plant Soil1986, 3: 176–180