At least ten software packages are available for marker-based detection and localization of loci contributing to quantitative traits in experimental animals and plants. Many of these have unique strengths or situations in which they are particularly useful. Six were developed by or in collaboration with plant geneticists and may not be well known to mammalian geneticists. These software packages are reviewed here and compared with a previously undescribed program, Map Manager QT, a Mac OS microcomputer program for mapping quantitative trait loci in populations derived from backcrosses, intercrosses, and recombinant inbred lines. Map Manager QT is an enhanced version of Map Manager Classic (Map Manager v2.6.5, Manly 1993), designed for mapping Mendelian loci. This review describes the methods Map Manager QT uses for mapping quantitative trait loci and describes other features that differ from those in Map Manager Classic. A complete description of both Map Manager Classic and Map Manager QT is available in the user manual, the on-line version of which can be found at http://mcbio.med.buffalo.edu/MMM/MMM.ht ml.