Cortex tissue from individuals of Mammillaria microcarpa was sampled for β-phenethylamines in 15 populations in Arizona. Qualitative and quantitative variation of these compounds were determined by HPLC analysis. Tyramine and N-methyltyramine were found in all plants, whereas hordenine and dimethyldopamine occurred in 95% and 64% of the samples, respectively. Statistical analyses yielded no clear and significant association, qualitatively or quantitatively, with the geographical distribution of the populations. We conclude that the three major β-phenethylamines are stable and reliable taxonomic characters in M. microcarpa.