Movement of pesticides on soil thin-layer chromatographic (TLC) plates was detected by a bioassay employing a green alga, Chlorella sorokiniana Shihira and Krauss. After leaching the soil TLC plates with water, a Chlorella suspension in an enriched agar medium was aspirated onto the soil to give ca. 8 × 106 cells/cm2. Plates were incubated at 100% relative humidity at 30 C, with illumination of ca. 40 klux. Zones of pesticide movement were visible within 24 to 48 hr. Mobility in Hagerstown silty clay loam was assessed for two methylcarbamate insecticides and for nine phenylurea, 11 s-triazine, and 13 miscellaneous herbicides. Among analogous triazines, the mobility order was: -OCH3 > -Cl > -SCH3. Movement of 6,7-dihydrodipyrido[1,2-a:2′,1′-c]pyrazinediium ion (diquat) and 1,1′-dimethyl-4,4′-bipyridinium ion (paraquat), normally immobile in soils, was directly proportional to rate when applied in high doses.