The average Y-STR mutation rate for the combined Texas populations is approximately 2.1 x 10 to the minus-three power per locus (95 percent CI [1.7-2.5] x 10 to the minus three power), which is comparable to other studies; however, taking all observed mutations together, Y-STR mutation rates differed between African-Americans and those of Caucasians and Hispanics, with the African-Americans being significantly higher. Most mutations resulted in one-step repeat differences consistent with the strand slippage model for generating mutations with STRs. No bias for gains or losses of repeats was observed, which differs from previous studies. Mutation rate correlates with the length of alleles, with longer alleles being more likely to mutate compared with shorter alleles. When the mutations were grouped by absolute repeat sizes of the progenitor alleles (i.e., fathers' allele sizes), interoperation differences in mutation rates became insignificant, while the positive correlation between mutation rate and allele size remained significant. Mutations across the different Y-STR loci were apparently independent of each other. DNA was obtained from unrelated male donors and their confirmed sons, based on paternity testing cases submitted to the DNA Identification Lab at the University of North Texas Health Science Center. The samples consisted of 950 African-American father-son pairs, 957 Caucasian father-son pairs; and 1,006 Hispanic father-son pairs. A detailed description is provided of the statistical analysis. 6 tables, 2 figures, and 20 references