The allelopathic effects of the exotic invasive weed, Johnsongrass, on Lactuca sativa, a native plant in China, were evaluated and the phytotoxins were investigated under laboratory conditions. The crude extracts (chloroform and ethyl acetate fractions) that were obtained from the ethanol extract of the subterranean parts of Johnsongrass inhibited the germination speed and growth of the roots and shoots of the test plant. The four compounds, ethyl p-hydroxybenzoate, diosmetin, apigenin, and luteolin, were isolated from the extract of the subterranean parts of Johnsongrass first, along with three other compounds (reported previously), p-hydroxybenzaldehyde, p-hydroxybenzoic acid, and dhurrin, and they all were evaluated on L. sativa. At the concentration of <0.5 mmol $L^{-1}$, ethyl p-hydroxybenzoate, dhurrin, and the mixture of the compounds delayed the germination speed of the seeds of L. sativa at 24 h of incubation, while apigenin delayed the germination speed of the seeds of L. sativa at 72 h of incubation. Ethyl p-hydroxybenzoate demonstrated the strongest delaying effect among the phenols and a similar effect was found with apigenin among the flavonoids, wheras ethyl p-hydroxybenzoate induced a drastic inhibition of the germination at 2 mmol $L^{-1}$. In contrast, p-hydroxybenzaldehyde and luteolin had no effect on the germination at any concentration at any stage. All the substances inhibited the shoot and root growth of L. sativa at 3 mmol $L^{-1}$. Increasing the concentration increased the inhibition of the growth of L. sativa.The inhibitory activity of ethyl p-hydroxybenzoate and p-hydroxybenzaldehyde was greater than that of the other compounds. This result suggested that the isolated phytotoxins might contribute to the successful invasion by Johnsongrass.