The Adh-1 null mutant (B15-1-8) isolated previously was used to establish conditions that allow the selection of ADH-deficient pollen grains and seeds of tomato. New Adh-1 null mutants were then selected among the progenies derived from crosses between the genetically unstable tomato lines Yvms, Sulfurea and 2s-Iso and the Adh-1 null B15-1-8 mutant. Treatment of 158 500 F1 seeds with allyl alcohol led to the identification of a new Adh-1 null mutant (OYO 424). Similarly, fertilization of B15-1-8 with allyl alcohol-treated pollen collected from the mutable lines resulted in the recovery of another Adh-1 null mutant (BOY 421). Like the mutant B15-1-8, OYO 424 and BOY 421 were found to lack Adh-1 activity in both pollen and seeds. Genetic analysis showed that the mutations in both mutants were allelic to the Adh-1 locus on chromosome 4. Southern analysis, using as a probe Adh-1 genomic clones spanning the entire coding region and 2 kb of the upstream region, suggested that the mutations of the Adh-1 alleles in B15-1-8, OYO 424 and BOY 421 were due to small alterations and not to the insertion of a transposable element. Remarkably, the mutant allele of B15-1-8 was highly unstable, reverting to the wild type allele at a frequency of 5 x 10−4.