To study the isonymy structure of Texas, we analyzed the surname distributions of 3.6 million telephone users registered for the year 1996 in 232 towns distributed in the 7 regions of the state. The number of different surnames was 235,740. Matrices of isonymy distances between towns and between geographic regions were constructed and tested for correlation with geographic distance. We found that isonymy distances between the seven regions showed borderline or no correlation with geographic distance, with r = 0.089 ±0.232, r = 0.492 ±0.232, and r = 0.337 ±0.232 for Lasker's, Euclidean, and Nei's distances, respectively. Isonymy distances between towns were significantly correlated with geographic distance, with r = 0.249 ±0.006 for Lasker's distance, r = 0.338 ±0.006 for the Euclidean distance, and r = 0.418 ±0.006 for Nei's distance. Two dendrograms, one for the 7 regions and one for the 232 towns, were built from the matrices of Nei's distances. The dendrogram for regions indicates that a main surname differentiation exists between the East and West areas of Texas, with West Texas being predominantly Hispanic and East Texas being predominantly English-speaking. The dendrogram for the towns confirms in detail the differences identified by the matrix of distances between regions. Random inbreeding calculated from isonymy, F_ST, was highest in the west and in the south of the state. It was lowest in the area of Austin and Houston. Average Fisher's αfor towns was 734, for regions it was 1,047, and for Texas as a whole it was 1,230. The geographic distribution of αin the state shows distinctly lower values in the traditionally Hispanic west and higher values in the east and on the Gulf of Mexico.