Electric-field effects on excitons in zero-dimensional InGaAs quantum disks have been examined at low temperature. Photoluminescence from a single isolated disk was measured under the application of a lateral electric field by using the microphotoluminescence technique. A redshift of sharp excitonic luminescence and a decrease in its intensity under increasing electric field were observed. These were found to distinctively depend on the lateral extent of the disks: these were much more prominent in the larger disk. The exciton luminescence was found to be highly polarized along the direction of the field in the larger disk. (C) 2000 American Institute of Physics. [S0003-6951(00)00207-2].