Owing to increasing scientific and agricultural interest in the disease-resistant (trypanotolerant), indigenous cattle breeds of West and Central Africa, there is a need for a rational genetically based description of populations in the region. The greatest threat to the invaluable genetic resource represented by these animals is that of extensive genetic introgression of distantly related zebu cattle from northern populations which do not share their inherited tolerances. Southern blotting with a chromosome Y-specific probe, btDYZ-1 (locus DYZ1) is shown to be a sensitive assay to detect such introgression. Evidence of historical crossbreeding is reported in two important N'Dama populations previously classed as purely taurine.