`Jeffries', a mutant of `Nonpareil' almond [Prunus dulcis (Mill.) D.A. Webb], showed “unilateral incompatibility” in that its pollen failed to fertilize cultivars in the `Carmel' (CIG-V), `Monterey' (CIG-VI), and `Sonora' (CIG-VII) pollen cross-incompatibility groups (CIGs), as well as specific cultivars (`Butte', `Grace', and `Valenta') whose CIG group is unknown. `Jeffries' is not self-compatible, but produced good set when pollinated by 12 almond cultivars representing the entire range of CIGs involving `Nonpareil' parentage, as well as the parent `Nonpareil'. It was concluded that the `Jeffries' mutant—both gametophyte and sporophyte—expressed a loss of a single S allele of the `Nonpareil' genotype.