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To clarify incompatibility relationships among almond cultivars, 35 were analysed for stylar ribonucleases, which have previously been shown to correlate with incompatibility S alleles. Stylar proteins were extracted and separated electrophoretically and the zymograms compared with ladders of ribonucleases corresponding to the 12 S alleles previously reported. Sixteen cultivars showed a band corresponding to two of the known ribonucleases, 17 showed one known ribonuclease and one ‘new’ band, and two showed two new bands. Twelve new ribonucleases were detected; 11 were attributed to new S alleles (S13 to S23) and a mutant form of S7 was attributed to S7A. Genotypes were proposed for nine cultivars of five incompatibility groups that had not been genotyped previously, VII, X, XI, XII and XIII. Twenty‐four cultivars of unknown incompatibility relationships were provisionally genotyped: six of these could be assigned to existing groups and two new groups were established, XIV and XV, along with group O of cultivars with unique genotypes. Test crosses confirmed that eight pairs of cultivars showing similar zymograms were indeed cross‐incompatible, including the two representatives of each of the two new groups. Virtually all self‐incompatible cultivars of known genotype are listed in a table. The data should be useful for planning cultivar combinations for orchards and for designing crosses for breeding programmes.

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Identification of the incompatibility genotypes of almond cultivars is important in breeding programmes for designing crosses and for selecting progeny. This paper describes a novel molecular technique for the identification of S‐alleles in almond based on the use of PCR primers designed from the sequences of the introns without the need for restriction enzyme digestion. Nine specific pairs of primers have been designed for the S1, S2, S5, S7, S8, S9, S10 (putative), S23 and Sf alleles, and these confirmed the S‐allele specificities for 22 of the 23 accessions for which published information is available. This technique provides a precise method for identifying S‐alleles from the genomic DNAs of almond cultivars, and will be useful for confirming the segregation of alleles in breeding progeny.