In an intercropping system with leek (Allium porrum L.) and celery (Apium graveolens L. var. dulce (Mill.) Pers.), weed suppression is improved through increased canopy light interception. Intra- and interspecific competition in the system, however, affects the performance of the crops with respect to yield and quality. The objective of the study was to quantify intra- and interspecific competition by leek and celeriac [Apium graveolens L. var. rapaceum (Mill.) Gaud.-Beaup] or celery in an intercropping system. A 3 yr study was carried out to investigate the effects of plant density, relative proportion of component crop, spatial arrangement, and N input on biomass production, crop quality, and N use in an intercropping system with leek and celeriac or celery. Land equivalent ratios exceeding unity were found, indicating an improved resource use by the crop mixture. Relative yield totals around one showed that with respect to biomass production, no yield advantage was found in the crop mixture. Analyses using a hyperbolic yield density response model showed that the competitive ability of celeriac and celery was significantly higher than that of leek. Effects of intra- and interspecific competition resulted for both crops in a reduction of the quality. Nitrogen utilization efficiency (E U) was generally poor in all crop stands, particularly at a high N application rate. The intercropping system needs improvement with respect to crop quality, and it is suggested to apply ecophysiological crop growth models to maximize crop complementarity and competitive ability against weeds.