You must be populating drop down list on page_load but not inside ispostback condtion thats y each time the page is post back the list will be populated with the data again
supose XYZ... is ur code for populating dropdownlist then use this
page_load()
{
if(!IsPostBack)
{
XYZ...
}
}
hope ur problem cud be resolved with this
Regards,
Rashida
www.akaas.net