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pattern for each enzyme (Table 2, Appendix I). Statistical analysis.- Several samples were excluded from statistical analyses due to small sample sizes (e.g., CRIT canals, Little Colorado River). Because complete restriction fragment profiles were not obtainable for all individuals (Table 2, Appendix I), two separate sets of analyses were performed, one using the subset of individuals with composite haplotypes for all enzymes and the other including all individuals with composite haplotypes for only six base-pair (bp) enzymes. Because of its degenerate recognition sequence, BanI may cleave the same site as HinPI (when Py=C and Pu=G); information from BanI therefore was excluded from both distance calculations and phylogenetic analysis. Levels of genetic diversity within samples were assessed by direct count of haplotypes (i.e., alleles) corrected for sample size (nh) and estimates of haplotype diversity (h). Haplotype diversity (Nei and Tajima, 1981) was calculated using the program REAP (version 4.0, McElroy et al., 1992). This value is frequently used to assess levels of mtDNA diversity within a sample of individuals (e.g., Avise, 1992), providing a measure of the relative contribution of each haplotype to the sample. High values (maximum=l.0) are found when each genotype is equally represented. Low values (minimum=0.0) occur when specific genotypes are over-represented. Geographic heterogeneity in allele frequencies was also examined by Monte Carlo simulation (Roff and Bentzen, 1989), using the program REAP. In the Monte Carlo approach, a X2 test is performed to test for heterogeneity in the distribution of hapIotypes among populations. Because of the typically small cell sizes in such analyses, it is necessary to evaluate the power of the test by contrasting this value with x2 values calculated from 1000 new population by haplorype matrices produced by randomly sampling (with replacement) the original data matrix. An original test value greater than those c~culated 5