The incidence of supernumerary teats has been recorded in 179,793 German Simmental and 37,460 German Brown Swiss cows. Data were collected from first-crop daughters of test bulls from 1987 to 1998. The number of sires was 4,298 and 1,039, respectively. The average frequency of affected animals was 44.3% in German Simmental and 31.2% in German Brown Swiss. A significant yet small effect was found for herd-book membership (yes/no) and, in the Simmental data only, for inspector. The impact of year of birth and year of inspection was also significant, and more important, reflected a decrease of the population averages with time. Surprisingly, the incidence of supernumerary teats increased significantly with the parity number of dams in both breeds. A Bayesian threshold animal model approach was used to estimate the heritability of the occurrence of supernumerary teats. The posterior mean for the heritability was h2 = 0.45 in German Simmental and h2 = 0.43 in Brown Swiss, with standard errors of 0.01 and 0.03, respectively. Ranks of sires obtained from threshold and linear models showed a rank correlation of roughly 0.8 in both breeds. For a sound identification of the worst sires, a threshold model is recommended.