Tomato plants were transformed with a chimaeric polygalacturonase (PG) gene, designed to produce a truncated PG transcript constitutively. In these plants expression of the endogenous PG gene was inhibited during ripening, resulting in a substantial reduction in PG mRNA and enzyme accumulation. This inhibition was comparable to that achieved previously using antisense genes. The expression of the truncated gene in ripe fruit was substantially lower than its expression in green fruit. Thus expression of both the endogenous and truncated genes is reduced in ripe fruit in which both are active. The implication of this observation is discussed in relation to the possible mechanism whereby sense constructs inhibit gene expression.