The Betuleae is one of the two Betulaceae tribes and contains two genera, the actinorhizal genus Alnus and the nonactinorhizal genus Betula. To elucidate the phylogenetic relationships within the tribe and compare the relative amounts of genetic diversity between the genera Alnus and Betula, rbcL and 18S rRNA gene sequences as well as rDNA internal transcribed spacer (ITS1 and ITS2) sequences were obtained and compared. In rbcL and (18 S) rRNA, several substitutions were detected between Alnus and Betula but no variation was observed within Alnus. Minimal length variation was observed among ITS sequences. The numbers of substitutions per site for ITS were, on average, close to 10 times those estimated for (18 S) between Alnus and Betula. In ITS, numbers of substitutions were, on average, more than twice as large between genera than within genera. Phylogenies from ITS sequences were estimated using neighbor-joining analysis of substitution rates, maximum likelihood and parsimony methods. Each of the two genera appeared monophyletic, which was supported by high bootstrap estimates. Relationships among the three sections of Betula analyzed were also better resolved than those among the three subgenera of Alnus analyzed. Phylogenies estimated from ITS sequences suggested a relatively recent diversification of each genus, in agreement with fossil data. (\quad 1993) Academic Press. Inc.