Sub Test()
Dim WsS As Worksheet
Dim LigneS As Long
Set WsS = Worksheets("Feuil1")
For LigneS = 3 To WsS.Range("A" & Rows.Count).End(xlUp).Row
If WsS.Cells(LigneS, 2) = "" Then
WsS.Cells(LigneS, 1) = WsS.Cells(LigneS - 1, 1)
End If
Next LigneS
End Sub