In previous work, a modified version of the Bayesian information criterion (mBIC) was proposed to locate multiple interacting quantitative trait loci (QTL). Simulation studies and real data analysis demonstrate good properties of the mBIC in situations where the error distribution is approximately normal. However, as with other standard techniques of QTL mapping, the performance of the mBIC strongly deteriorates when the trait distribution is heavy tailed or when the data contain a significant proportion of outliers. In the present article, we propose a suitable robust version of the mBIC that is based on ranks. We investigate the properties of the resulting method on the basis of theoretical calculations, computer simulations, and a real data analysis. Our simulation results show that for the sample sizes typically used in QTL mapping, the methods based on ranks are almost as efficient as standard techniques when the data are normal and are much better when the data come from some heavy-tailed distribution or include a proportion of outliers. (2006). The most difficult part in constructing an appropriate regression model is the decision on the number of its components (i.e., the QTL number). This decision is particularly important in cases where the trait is influenced by some linked QTL. In such situations, the estimated QTL locations may depend substantially on the number of QTL in the model. When the size of the model is underestimated, for example, two linked QTL may be easily represented as one putative QTL in the middle between two real QTL locations. The opposite situation occurs when the size of the model is overestimated, leading to the incorrect identification of spurious QTL.Since the size of the chosen model depends on the level of significance used for including or deleting its components, the choice of the corresponding threshold value may substantially influence the results of the analysis. The corresponding problem exists also in the framework of Bayesian statistics, where the final estimates of the QTL locations depend on the prior distribution of the QTL number. In the setting of classical statistics, a systematic approach for the comparison of different models is provided by model selection criteria. Different model selection criteria serve different purposes. If the purpose of the study is the choice of markers for marker-assisted selection, then one should consider the criteria aiming at minimizing the prediction error, like, e.g., the Akaike information criterion (Akaike 1974). Note that the prediction does not suffer much from including several markers closely linked to a QTL. However, in the case when the purpose of the study is to identify real QTL, then consistent criteria, like, e.g., the Bayesian information criterion (BIC) (Schwarz 1978), seem to be a better choice.The classical model selection criteria were developed on the basis of asymptotic arguments and assuming that the sample size is large in comparison to the size of the analyzed models. This assumption is no longer...