Within the cactus genusMammillaria, the circumscription of series Stylothelae has been controversial. This has resulted in circumscriptions that group species into a single series with various species groups (M. crinita Group, M. bombycina Group, M. glassii Group, and M. mercadensis Group) and circumscriptions that split the series into a number of subsequent series. Utilizing PCR sequencing methods, the authors investigated phylogenetic relationships in the Stylothelae and discovered the deletion of the entire rpl16 intron of the chloroplast in a number of members of the series. To date, this deletion has not been documented elsewhere in the Cactaceae. Specifically, the deletion of the rpl16 intron in the Stylothelae is restricted to those species that have either been treated as a distinct species group (M. crinita Group) within the series, or as a unique series in their own right (series Stylothelae sensu stricto).