Six cellulase genes were isolated from total RNA of the ethylene-treated tomato (Lycopersicon esculentum Mill.) flower abscission zone by reverse-transcription polymerase chain reaction using degenerate primers to conserved amino acid sequences from known plant cellulases. Four of the gene fragments are homologous to fruit pericarp cellulases. The other two are novel cellulase genes, referred to as Cel5 and Cel6. Breakstrength and cellulase gene expression were then analyzed in naturally abscising flowers and flower explants. In both naturally abscising flowers and flower explants induced to abscise in air or ethylene, both new cellulase mRNAs were correlated with flower shedding. Whereas the Cel5 mRNA increased in later stages of abscission, the Cel6 mRNA was present in nonabscising flowers and then decreased in the final stage of abscission. A third cellulase, Cel1, increased during the final stage of abscission in flower explants and yet did not increase during shedding in planta, although it was detectable at low levels in all abscission stages. Cel1 and Cel5 mRNA decreased 99% when indole-3-acetic acid was added during ethylene treatment, consistent with low levels of abscission (3%). In contrast, Cel6 mRNA increased slightly when indole-3-acetic acid was added. These results suggest that abscission is a multistep process involving both activated and repressed cellulase genes and that the relative importance of each cellulase in the process depends on the physiological conditions under which abscission takes place.