Dim c As Range, plage As Range
Dim b As Boolean
Columns("c").Clear
For Each c In Range("a1:a" & Range("a65536").End(xlUp).Row)
If Columns("b").Find(c) Is Nothing Then
If Not b Then Set plage = c: b = Not b Else Set plage = Union(plage, c)
End If
Next c
plage.Copy Range("c1")