Changing Ranges in VBA

Dear all,

I have written the following code which replaces "" (a blank) with a zero, yet it applies to the whole workbook. How would I change this to apply it to the active sheet of range A1 to J815209?

Sub ReplaceCharacter()
For Each mySheet In ThisWorkbook.Worksheets
mySheet.Cells.Replace What:="", Replacement:=0, LookAt:=xlPart, SearchOrder:=xlByRows, MatchCase:=False, SearchFormat:=False, ReplaceFormat:=False
Next
End Sub

Thanks
smurray444

Try to change your code:

Sub ReplaceCharacter()
   lastrow = cells(rows.count,"j").end(xlup).row
   activeSheet.range("a1:j" & lastrow).Replace What:="", Replacement:=0, LookAt:=xlPart,     SearchOrder:=xlByRows, MatchCase:=False, SearchFormat:=False,   ReplaceFormat:=False
End Sub

Regards,
Antonio