//Note that you need to set the PrimaryKey property of a DataTable in order to use Merge. DataSet dsRefreshData = new DataSet(); MyDataAdapter.Fill(dsRefresh); dsRefresh.Tables[0].PrimaryKey = new DataColumn[1] {dsRefresh.Tables[0].Columns["MyPKColumn"]}; // Assuming that the tables in dsMyData also have PrimaryKeys set dsMyData.Merge(dsRefresh);~~Bonnie