diff --git a/src/write_global_output.f90 b/src/write_global_output.f90
index 15dcf6af11ab674b0078ebc74be9e4bd02747e61..148dba7c0f5741fd3cac867238810f8c0c0d3f64 100644
--- a/src/write_global_output.f90
+++ b/src/write_global_output.f90
@@ -185,6 +185,7 @@ if (iproc.eq.0) then
 
    if (params%isobc) then 
      ! write isostasy basal displacement array - dwhipp 11/09 
+       write (9) 2**params%levelmax_oct
        write (9) ((zi%zisodisp(i,j),j=1,2**params%levelmax_oct+1),i=1,& 
                  2**params%levelmax_oct+1) 
    end if