Experiments were conducted to explore differential interaction of Rhizoctonia solani AG 2-t isolates on tulip cultivars in soil artificially infested under different experimental conditions. Comparison of residual variances obtained by analysis of variance and by analysis of additive main effects and multiplicative interaction effects (AMMI) showed that open-air experiments should be used for interpretation of isolate by cultivar interaction. In open-air experiments, variability was lower than in greenhouse tests. In the hiplot, derived after AMMI-analysis over isolates by years and by cultivars, isolates tended to occur in year clusters, indicating a differential effect of year on disease expression. Three isolates occurred in isolate clusters, thus accounting for a significant year by isolate by cultivar interaction. One cluster consisted of three isolates high in aggressiveness on all tested tulip cultivars, and another cluster consisted of three isolates low in aggressiveness. Greenhouse conditions and inoculum carrier significantly influenced quantitative differential interaction patterns. Isolates grown on oat kernels were more aggressive than those grown on autoclaved perlite particles soaked in liquid malt peptone. Differential interaction of AG 2-t isolates on tulip cultivars does occur, although it cannot yet be disentangled completely.