subroutine output_header(filename)
  use amr_commons
  use hydro_commons
  use pm_commons
  implicit none
  include "mpif.h"
  integer::info
  character(LEN=*)::filename

  integer::ilun
  integer::npart_family_loc(-5:5), npart_family(-5:5), npart_all_loc, npart_all
  integer :: ifam, ipart

  if(verbose)write(*,*)'Entering output_header'

  ! Compute total number of particles
  ! Count number of particles
  npart_family_loc = 0; npart_all_loc = 0
  [ doing some stuff ]

  ! myid contains the id of the thread
  if (myid == 1) then
     ! Open file
     open(newunit=ilun,file=trim(filename),form='formatted',status='replace')

     write(ilun, '(a1,a12,a10)') '#', 'Family', 'Count'
     do ifam = -NFAMILIES, NFAMILIES
        write(ilun, '(a13, i10)') &
             trim(particle_family_keys(ifam)), npart_family(ifam)
     end do
     write(ilun, '(a13, i10)') &
          'undefined', npart_all - sum(npart_family)

     [ do more stuff here ]
     close(ilun)

  endif

end subroutine output_header