Abstract The phytotoxic activity of celery ( Apium graveolens L.) was tested on germination and seedling growth of Lactuca sativa (lettuce). Bioactive compounds were isolated and identified by spectral data. Methanol extract of celery aerial part showed a significant phytotoxic effect on lettuce. Thus, this extract was divided into hexane and methanol fractions. Bioassay-directed of these two fractions led to the isolation and characterization of six secondary metabolites which included three phthalides (senkyunolide A, ( 3S )-butylphthalide and sedanolide), two furanocoumarins (bergapten and scopoletin) and one phenylpropanoid ( p -hydroxyphenethyl trans -ferulate). Among these compounds, senkyunolide A was the most toxic on lettuce germination and shoot growth. However, p -hydroxyphenethyl trans -ferulate was the most toxic on root growth.