Sex determination in cucumber (Cucumis sativus ) plants is genetically controlled by the F and M loci. The F and M loci interact to produce three different sex phenotypes: gynoecious (M-F-), monoecious (M-ff), and andromonoecious (mmff). We found that the levels of ethylene production and the accumulation of CS-ACS2 mRNA in andromonoecious cucumber plants did not differ from those in monoecious plants and were less than the levels measured in gynoecious plants. Ethylene inhibited stamen development in monoecious cucumber but not in andromonoecious one. To investigate the action mechanism of ethylene in the induction of femaleness of cucumber flowers, we isolated three ethylene-receptor-related genes, CS-ETR1, CS-ETR2 and CS-ERS. Ethylene caused substantial increases in the accumulation of CS-ETR2, CS-ERS, and CS-ACS2 mRNA in monoecious cucumber plants, but not in andromonoecious plants. These results suggest that ethylene responses in andromonoecious cucumber plants are reduced compared to those in monoecious plants. This is the first evidence that ethylene signals may mediate the product of the M locus to inhibit stamen development in cucumber. The andromonoecious line provides novel material to study the function of the M locus during sex determination of flowers in cucumber plants.