Pathogenicity and host-parasite relationships in root-knot disease of celery (Apium graveolens ) caused by Meloidogyne incognita race 1 were studied under glasshouse conditions. Naturally and artificially infected celery cv. D’elne plants showed severe yellowing and stunting, with heavily deformed and damaged root systems. Nematode-induced mature galls were spherical and/or ellipsoidal and commonly contained more than one female, males and egg masses with eggs. Feeding sites were characterized by the development of giant cells that contained granular cytoplasm and many hypertrophied nuclei. The cytoplasm of giant cells was aggregated along their thickened cell walls and consequently the vascular tissues within galls appeared disrupted and disorganized. The relationship between initial nematode population density (Pi) and growth of celery plants was tested in glasshouse experiments with inoculum levels that varied from 0 to 512 eggs and second-stage juveniles (J2) mL−1 soil. Seinhorst's model y = m + (1 – m)zP–T was fitted to height and top fresh weight data of the inoculated and control plants. The tolerance limit with respect to plant height and fresh top weight of celery to M. incognita race 1 was estimated as 0·15 eggs and J2 mL−1 soil. The minimum relative values (m) for plant height and top fresh weight were 0·37 and 0·35, respectively, at Pi ≥ 16 eggs and J2 mL−1 soil. The maximum nematode reproduction rate (Pf/Pi) was 407·6 at an initial population density (Pi) of 4 eggs and J2 mL−1 soil.