diff --git a/sacc/tracers.py b/sacc/tracers.py
index 161c290..5f501a8 100644
--- a/sacc/tracers.py
+++ b/sacc/tracers.py
@@ -652,7 +652,7 @@ class QPNZTracer(BaseTracer, tracer_type='QPNZ'):
     """
     A Tracer type for tomographic n(z) data represented as a `qp.Ensemble`
 
-    Takes a `qp.Ensemble`
+    Takes a `qp.Ensemble` and optionally a redshift array.
 
     Requires the `qp` and `tables_io` packages to be installed.
 
@@ -666,7 +666,7 @@ class QPNZTracer(BaseTracer, tracer_type='QPNZ'):
         The qp.ensemble in questions
     """
 
-    def __init__(self, name, ens, **kwargs):
+    def __init__(self, name, ens, z=None, **kwargs):
         """
         Create a tracer corresponding to a distribution in redshift n(z),
         for example of galaxies.
@@ -680,6 +680,11 @@ def __init__(self, name, ens, **kwargs):
         ensemble: qp.Ensemble
             The qp.ensemble in questions
 
+        z: array
+            Optional grid of redshift values at which to evaluate the ensemble.
+            If left as None then the ensemble metadata is checked for a grid.
+            If that is not present then no redshift grid is saved.
+
         Returns
         -------
         instance: NZTracer object
@@ -687,7 +692,16 @@ def __init__(self, name, ens, **kwargs):
         """
         super().__init__(name, **kwargs)
         self.ensemble = ens
-
+        if z is None:
+            ens_meta = ens.metadata()
+            if 'bins' in list(ens_meta.keys()):
+                z = ens_meta['bins'][0]
+        self.z = z
+        if z is None:
+            self.nz = None
+        else:
+            self.nz = np.mean(ens.pdf(self.z),axis=0)
+        
     @classmethod
     def to_tables(cls, instance_list):
         """Convert a list of NZTracers to a list of astropy tables
@@ -710,6 +724,16 @@ def to_tables(cls, instance_list):
         tables = []
 
         for tracer in instance_list:
+            if tracer.z is not None:
+                names = ['z', 'nz']
+                cols = [tracer.z, tracer.nz]
+                fid_table = Table(data=cols, names=names)
+                fid_table.meta['SACCTYPE'] = 'tracer'
+                fid_table.meta['SACCCLSS'] = cls.tracer_type
+                fid_table.meta['SACCNAME'] = tracer.name
+                fid_table.meta['SACCQTTY'] = tracer.quantity
+                fid_table.meta['EXTNAME'] = f'tracer:{cls.tracer_type}:{tracer.name}:fid'
+
             table_dict = tracer.ensemble.build_tables()
             ap_tables = convertToApTables(table_dict)
             data_table = ap_tables['data']
@@ -731,6 +755,8 @@ def to_tables(cls, instance_list):
                 meta_table.meta['META_'+kk] = vv
             tables.append(data_table)
             tables.append(meta_table)
+            if tracer.z is not None:
+                tables.append(fid_table)
             if ancil_table:
                 ancil_table.meta['SACCTYPE'] = 'tracer'
                 ancil_table.meta['SACCCLSS'] = cls.tracer_type
@@ -775,6 +801,10 @@ def from_tables(cls, table_list):
 
         for val in sorted_dict.values():
             meta_table = val['meta']
+            if 'fid' in val:
+                z = val['fid']['z']
+            else:
+                z = None
             ensemble = qp.from_tables(val)
             name = meta_table.meta['SACCNAME']
             quantity = meta_table.meta.get('SACCQTTY', 'generic')
@@ -783,7 +813,7 @@ def from_tables(cls, table_list):
             for key, value in meta_table.meta.items():
                 if key.startswith("META_"):
                     metadata[key[5:]] = value
-            tracers[name] = cls(name, ensemble,
+            tracers[name] = cls(name, ensemble, z=z,
                                 quantity=quantity,
                                 metadata=metadata)
         return tracers
diff --git a/test/test_sacc2.py b/test/test_sacc2.py
index 54fb09f..a50379a 100644
--- a/test/test_sacc2.py
+++ b/test/test_sacc2.py
@@ -833,10 +833,10 @@ def test_qpnz_tracer():
     nz_qp_interp = qp.Ensemble(qp.interp, data=dict(xvals=z, yvals=np.ones(shape=(1, 101))))
     nz_qp_hist = qp.Ensemble(qp.hist, data=dict(bins=z, pdfs=np.ones(shape=(1, 100))))
 
-    T1 = sacc.BaseTracer.make('QPNZ', 'tracer1', nz_qp_interp,
+    T1 = sacc.BaseTracer.make('QPNZ', 'tracer1', nz_qp_interp, z,
                               quantity='galaxy_density',
                               metadata=md1)
-    T2 = sacc.BaseTracer.make('QPNZ', 'tracer2', nz_qp_hist,
+    T2 = sacc.BaseTracer.make('QPNZ', 'tracer2', nz_qp_hist, z,
                               quantity='galaxy_shear',
                               metadata=md2)
     assert T1.metadata == md1
@@ -850,6 +850,15 @@ def test_qpnz_tracer():
     assert T1a.metadata == md1
     assert T2a.metadata == md2
 
+    # test version without saved z
+    T3 = sacc.BaseTracer.make('QPNZ', 'tracer3', nz_qp_interp, 
+                              quantity='galaxy_density',
+                              metadata=md1)
+    tables = sacc.BaseTracer.to_tables([T3])
+    D = sacc.BaseTracer.from_tables(tables)
+    assert D['tracer3'].z is None
+
+
 
 def test_io_qp():
     s = sacc.Sacc()
@@ -859,7 +868,7 @@ def test_io_qp():
     nz = np.expand_dims((z-0.5)**2/0.1**2, 0)
     ens = qp.Ensemble(qp.interp, data=dict(xvals=z, yvals=nz))
     ens.set_ancil(dict(modes = ens.mode(z)))
-    s.add_tracer('QpnZ', 'source_0', ens)
+    s.add_tracer('QpnZ', 'source_0', ens, z)
 
     for i in range(20):
         ee = 0.1 * i