Summary: QTLNetwork is a software package for mapping and visualizing the genetic architecture underlying complex traits for experimental populations derived from a cross between two inbred lines. It can simultaneously map quantitative trait loci (QTL) with individual effects, epistasis and QTL–environment interaction. Currently, it is able to handle data from F2, backcross, recombinant inbred lines and double-haploid populations, as well as populations from specific mating designs (immortalized F2 and BCnFn populations). The Windows version of QTLNetwork was developed with a graphical user interface. Alternatively, the command-line versions have the facility to be run in other prevalent operating systems, such as Linux, Unix and MacOS. Availability: http://ibi.zju.edu.cn/software/qtlnetwork Contact: jzhu@zju.edu.cn [ABSTRACT FROM AUTHOR]