Mutants of Arabidopsis thaliana with a glucosinolate content different from wild type were isolated by screening a mutagenized population of plants. Six mutants were detected out of a population of 1200 screened. One of these mutants, TU1, was analyzed in detail. Leaf and seed tissues of line TU1 lack or have reduced amounts of many of the aliphatic glucosinolates found in the wild type due to a recessive allele, gsm1, of a single nuclear gene, GSM1. The seed phenotype is inherited as a maternal effect suggesting that the embryo is dependent on the maternal tissue for its glucosinolates. Experiments involving feeding of ¹⁴C-labeled intermediates suggested that the gsm1 allele results in a metabolic block which decreases the availability of several amino acid substrates required for glucosinolate biosynthesis: 2-amino-6-methylthiohexanoic acid, 2-amino-7-methylthioheptanoic acid, and 2-amino-8-methylthiooctanoic acid. The mutation does not result in any obvious changes in morphology or growth rate. A pathway for the biosynthesis of glucosinolates in A. thaliana is proposed.