The determination of allelopathic affects from 12 perennial ryegrass, Lolium perenne L., cultivars was attempted. Field trials of overseeding crabgrass, Digitaria sanquinalis (L.) Scop., into ryegrass stands did not show differences among the 12 cultivars. Crabgrass overseeding trials with 99 other ryegrasses showed a range of crabgrass percent cover and inhibitions in bioassays. Laboratory investigations centered on the effects of extracts of leaf-stem tissue on growth of duckweed, Lemna minor L., fronds. Extracts were prepared by macerating frozen tissue samples in water, centrifuging and filtering. Full (10 g/30 ml), one-half and one-quarter strength extract concentrations generally resulted in decreasing inhibition of frond growth, but sometimes stimulations occurred. Various bioassays of crabgrass seed germination showed some inhibitions. Overall results as to inhibition of duckweed and crabgrass by specific cultivars were inconsistent.