Bonjour anne69,
Si j'ai bien compris :
Option Explicit
Sub test()
Dim r As Range, i As Long, j As Long
Application.ScreenUpdating = False
With Sheets("feuil1")
Set r = .Range("a2", .Cells(.Rows.Count, "a").End(xlUp))
For i = 1 To r.Count
j = 1
Do Until r(i) <> r(i).Cells(j)
j = j + 1
Loop
r(i).Offset(, 2).Value = Application.Sum(.Range(r(i), r(i).Cells(j - 1)).Offset(, 1))
i = i + j - 2
Next i
End With
Application.ScreenUpdating = True
End Sub
klin89