-Nine populations representative of section Erythranthe and twelve of their interpopulation F1 hybrids were analyzed by starch gel electrophoresis for 11 enzyme systems: ACP, ADH, CAT, G6PDH, GDH, GOT, LAP, MDH, PGI, PGM and SKDH. A total of 32 alleles at 18 loci were detected in the group. The Erythranthe populations exhibited a low rate of allelic intrapopulation polymorphism, 4.9%, as well as a low rate of interpopulation polymorphism, 4.3%. Possibly, this pattern is due to rapid gene fixation in the small isolated populations so common in Erythranthe. Three computer programs, CONTML, BIOSYS, and PAUP, used in analyses of the results reveal four subgroups in section Erythranthe. The first subgroup consists of Mimulus cardinalis which appears to consist of two varieties, one from California and one from Arizona. The second subgroup consists of M. verbenaceus and M. nelsonii which are closely similar, possibly varieties of one species. The third subgroup consists of M. eastwoodiae and M. rupestris which are morphologically similar but allozymically clearly distinct. The fourth subgroup (sometimes divided in two) consists of the two races of M. lewisii, one from the Sierra Nevada and the other from the Rocky Mountains. These two races are so distinct from each other and from the rest of the section as to raise the question of their specific status. INTRODUCTION Over the years the analysis of isozymes and allozymes has become the standard method of ascertaining unbiased estimates of the genetic similarities and differences of the taxa of a group of organisms (Hubby and Throckmorton, 1965; Crawford and Wilson, 1979; Bruederle and Fairbrothers, 1986). Previously, we have carried out an isozyme/allozyme study of section Erythranthe of the genus Mimulus (Vickery and Wullstein, 1987) which, however, raised questions as to gene frequencies and as to the appositeness of the phenetic analysis of the results. The purpose of this study is to answer those questions by (1) increasing the sample size and thereby improving the accuracy of the gene frequency data and (2) critically comparing phenetic and cladistic analyses of the results. MATERIALS AND METHODS For this investigation we grew and studied nine populations that well represent the morphologic diversity and geographic range of section Erythranthe (Grant, 1924; Pennell, 1951; Hiesey et al., 1971; Vickery, 1978, 1984). In order to facilitate the comparison of the results with those of the earlier study (Vickery and Wullstein, 1987), we used the same set of populations as before. We grew the populations in the greenhouse from transplants, seeds collected in the wild or from seeds harvested from such plants. In addition we grew plants of 12 interpopulation F1 hybrids. These hybrids involved all nine of the study populations in one or more combinations. Also, we included a population of Mimulus guttatus (Table 1) as an outgroup for the analyses of relationships. For the electrophoretic investigations, we used vigorously growing, young leaves. Approximately 0.5 g of leaves per plant were gathered and immediately placed in precooled mortars on ice. The leaves were frozen in liquid nitrogen, ground, and the protein extracted with a sodium metabisulfite-germanium dioxide-mercaptoethanol buffer (Kelley and Adams, 1977; Mitton et al., 1979) in the approximate proportion of 2:1, grams of tissue to milliliters