diff --git a/src/input_xml.F90 b/src/input_xml.F90
index 5ad034231f..6a1854fa7b 100644
--- a/src/input_xml.F90
+++ b/src/input_xml.F90
@@ -97,33 +97,58 @@ contains
        path_cross_sections = trim(cross_sections_)
     end if
 
+    ! Make sure that either criticality or fixed source was specified
+    if (criticality_ % batches == 0 .and. fixed_source_ % batches == 0) then
+       message = "Number of batches on <criticality> or <fixed_source> " &
+            // "tag was zero."
+       call fatal_error()
+    end if
+
     ! Criticality information
-    if (criticality % batches > 0) then
+    if (criticality_ % batches > 0) then
+       ! Set run mode
        if (run_mode /= MODE_PLOTTING) run_mode = MODE_CRITICALITY
 
        ! Check number of particles
-       if (len_trim(criticality % particles) == 0) then
+       if (len_trim(criticality_ % particles) == 0) then
           message = "Need to specify number of particles per cycles."
           call fatal_error()
        end if
 
        ! If the number of particles was specified as a command-line argument, we
        ! don't set it here
-       if (n_particles == 0) n_particles = str_to_int(criticality % particles)
+       if (n_particles == 0) n_particles = str_to_int(criticality_ % particles)
 
-       ! Copy cycle information
-       n_batches     = criticality % batches
-       n_inactive    = criticality % inactive
+       ! Copy batch and generation information
+       n_batches     = criticality_ % batches
+       n_inactive    = criticality_ % inactive
        n_active      = n_batches - n_inactive
-       gen_per_batch = criticality % generations_per_batch
+       gen_per_batch = criticality_ % generations_per_batch
+    end if
 
-       ! Check number of active batches
-       if (n_active <= 0) then
-          message = "Number of active batches must be greater than 0."
+    ! Fixed source calculation information
+    if (fixed_source_ % batches > 0) then
+       ! Set run mode
+       if (run_mode /= MODE_PLOTTING) run_mode = MODE_FIXEDSOURCE
+
+       ! Check number of particles
+       if (len_trim(fixed_source_ % particles) == 0) then
+          message = "Need to specify number of particles per cycles."
           call fatal_error()
        end if
-    else
-       message = "Need to specify number of batches with <batches> tag."
+
+       ! If the number of particles was specified as a command-line argument, we
+       ! don't set it here
+       if (n_particles == 0) n_particles = str_to_int(fixed_source_ % particles)
+
+       ! Copy batch information
+       n_batches = fixed_source_ % batches
+       n_active  = fixed_source_ % batches
+    end if
+
+    ! Check number of active batches
+    if (n_active <= 0) then
+       message = "Number of active batches must be greater than 0."
        call fatal_error()
     end if
 
diff --git a/src/xml-fortran/templates/settings_t.xml b/src/xml-fortran/templates/settings_t.xml
index c882d33149..2d97a9494f 100644
--- a/src/xml-fortran/templates/settings_t.xml
+++ b/src/xml-fortran/templates/settings_t.xml
@@ -3,7 +3,7 @@
 
   <options rootname="settings" />
 
-  <typedef name="criticality_xml">
+  <typedef name="run_parameters_xml">
     <component name="batches" type="integer" default="0" />
     <component name="inactive" type="integer" />
     <component name="particles" type="word" length="25" default="''" />
@@ -35,11 +35,12 @@
     <component name="upper_right" type="double-array" />
   </typedef>
 
-  <variable name="criticality" type="criticality_xml" />
+  <variable name="criticality_" tag="criticality" type="run_parameters_xml" />
   <variable name="cross_sections_" tag="cross_sections" type="word" length="255" />
   <variable name="cutoff_" tag="cutoff" type="cutoff_xml" dimension="1" />
   <variable name="energy_grid_" tag="energy_grid" type="word" length="7" />
   <variable name="entropy_" tag="entropy" type="mesh_xml" dimension="1" />
+  <variable name="fixed_source_" tag="fixed_source" type="run_parameters_xml" />
   <variable name="no_reduce_" tag="no_reduce" type="word" length="3" />
   <variable name="ptables_" tag="ptables" type="word" length="3" />
   <variable name="seed_" tag="seed" type="integer" />