Isonymy in Venezuela was studied analyzing the surnames of 3.9 million adults 40 years and older, selected from the register of electors updated in 1991. The electors were not differentiated by sex and represented all 22 states of Venezuela and its Federal District. The states of Nueva Esparta, an island, and Mérida, in the Venezuelan Andes, have the highest coefficients of microdifferentiation (R(ST)) and of inbreeding due to random isonymy. The states of Miranda, Aragua, Carabobo, and the Federal District, which includes the capital, Caracas, or are very close to it, had the lowest R(ST) values. The weighted averages of Fisher's alpha, a measure of surname abundance, varied by state from 43 in Nueva Esparta to 226 in Miranda, with a value of 210 for the entire country, much smaller than those observed in Switzerland, Germany, or Italy (2,396, 2,855, and 5,855, respectively). The fact that 32% of the total Venezuelan sample has only 40 surnames easily accounts for this small alpha. The correlation between the Euclidean distance and the log of geographic distance between the capital cities of states in km is high and significant (r = 0.78). The dendrogram built with the Euclidean distance matrix indicates the presence of three main clusters. One is formed by 10 states located in the western-central part of the country. The states of Zulia and Falcón join this cluster at a distance of 0.54 Euclidean units. A second cluster is formed by Barinas and the three Andean states. After combining these two, a third cluster joins them, formed by six Eastern states. Finally, the state of Amazonas enters the dendrogram at a distance of 0.66 units. Am. J. Hum. Biol. 12:352-362, 2000. Copyright 2000 Wiley-Liss, Inc.