• The effects are shown here of genetic manipulation of invertase on fruit sugar metabolism in tomato. • Introgression of the acid invertase gene on chromosome 3 from Lycopersicon pimpinellifolium (an accession containing high invertase activity) into Lycopersicon esculentum (with relatively low activity) was used to study effects on fruit development and both sugar composition and content. • The L. pimpinellifolium parent fruit had higher fruit invertase activities, greater hexose contents and lower sucrose accumulation that the L. esculentum parent. A strong correlation between fruit invertase activity and soluble sugar content was observed only in the L. pimpinellifolium parent and not in the homozygous fruit from either cross. In all cases high soluble acid invertase activities prevented sucrose accumulation and led to the build-up of hexoses. • The introgression of the invertase locus from L. pimpinellifolium into L. esculentum did not result in higher soluble solids in the progeny than in the L. esculentum parent, but it did modify their relative composition. Changes in the invertase gene composition alone are not sufficient to increase fruit soluble solids in tomato.