A chemotaxis assay was developed to measure attraction or repulsion of infective juveniles of the root-knot nematodeMeloidogyne incognita to exudates of host roots. The assay was sufficiently sensitive to measure the repellent activity of a 25-μl sample of 7 mM NaCl. In tests of root exudate collected in a variety of ways from a variety of plants, avoidance responses were usually found but attraction was not. Extraction of the exudate from tomato with various organic solvents revealed that the repellent activity was highly polar. On Sephadex G-15 chromatography this exudate separated into two clearly defined peaks with apparent molecular weights of about 500 and 1000 daltons. The faster running peak (larger apparent size) contained much more activity than the other peak. Subsequent analysis of material from the larger peak via HPLC on a C18 column revealed a single peak of repellent activity. None of the chemical fractionations uncovered reproducible attractant activity.