WinLoki is a graphical interface under Microsoft Windows® for the Simon Heath’s program Loki. This program uses Bayesian methodologies to carry out QTL (Quantitative Trait Loci) mapping analyses giving estimates of the position, effect and number of QTLs involved. WinLoki shares with Loki its nice characteristics and has some useful utilities in a Windows® environment. The aim of WinLoki is to help researchers to the use of Loki without needing significant informatics background. The source code, executable files and user’s guides (in Spanish and English versions) are freely available at http://winloki. sourceforge.net/. The performance of WinLoki is illustrated via the analysis of a half-daughter pedigree of Spanish Friesian cattle on bovine chromosome 6 for the traits Fore Teat Placement and Rear Teat Placement.