The S-allele characterisation of 'Alzina' and 'Garondes', two local almond cultivars from the island of Majorca, by a multidimensional approach has allowed the confirmation of the presence of the S-f-allele and the identification of a new allele not previously described in almond S-36. When these cultivars were, phenotypically evaluated, both showed a self-incompatible phenotype and were cross-incompatible, as assessed by artificial self- and cross-pollinations and fruit sets after field pollinations, confirming that their S-f-allele is in its active form, S-f. Thus a new CGI group in almond is proposed and named XXVIII. These results confirm the wide diversity of S-alleles in almond both at genotypic and phenotypic levels, as well as their similarity with the S-alleles from other close Prunus species. This similarity suggests the possibility of allele introgression between species or allele identity by descent from a common ancestor.