diff --git a/csiborgtools/read/halo_cat.py b/csiborgtools/read/halo_cat.py
index 9674636..3bef45b 100644
--- a/csiborgtools/read/halo_cat.py
+++ b/csiborgtools/read/halo_cat.py
@@ -38,6 +38,8 @@ from .utils import add_columns, cols_to_structured
 from ..utils import fprint
 from .readsim import make_halomap_dict, load_halo_particles
 
+from collections import OrderedDict
+
 
 ###############################################################################
 #                           Base catalogue                                    #
@@ -61,7 +63,8 @@ class BaseCatalogue(ABC):
     _observer_velocity = None
     _mass_key = None
 
-    _cache = {}
+    _cache = OrderedDict()
+    _cache_maxsize = None
     _catalogue_length = None
     _is_closed = None
 
@@ -77,7 +80,7 @@ class BaseCatalogue(ABC):
                            ]
 
     def __init__(self, simname, nsim, nsnap, halo_finder, catalogue_name,
-                 paths, mass_key):
+                 paths, mass_key, cache_maxsize=64):
         self.simname = simname
         self.nsim = nsim
         self.nsnap = nsnap
@@ -89,6 +92,7 @@ class BaseCatalogue(ABC):
         self._data = File(fname, "r")
         self._is_closed = False
 
+        self.cache_maxsize = cache_maxsize
         self.catalogue_name = catalogue_name
 
     @property
@@ -168,6 +172,21 @@ class BaseCatalogue(ABC):
             raise RuntimeError("`data` is not set!")
         return self._data
 
+    @property
+    def cache_maxsize(self):
+        """Maximum size of the cache."""
+        if self._cache_maxsize is None:
+            raise RuntimeError("`cache_maxsize` is not set!")
+        return self._cache_maxsize
+
+    @cache_maxsize.setter
+    def cache_maxsize(self, cache_maxsize):
+        assert isinstance(cache_maxsize, int)
+        self._cache_maxsize = cache_maxsize
+
+    def cache_length(self):
+        return len(self._cache)
+
     @property
     def observer_location(self):
         """Observer location."""
@@ -440,6 +459,7 @@ class BaseCatalogue(ABC):
         if "snapshot" in key:
             return self.data[key]
 
+        # For non-snapshot keys, we cache the results.
         try:
             return self._cache[key]
         except KeyError:
@@ -466,23 +486,24 @@ class BaseCatalogue(ABC):
                 out = numpy.linalg.norm(out - self.observer_location, axis=1)
             elif key == "angular_momentum":
                 out = numpy.vstack([self["Lx"], self["Ly"], self["Lz"]]).T
-            elif key in self.data[self.catalogue_name].keys():
-                out = self.data[f"{self.catalogue_name}/{key}"][:]
             elif key == "particle_offset":
                 out = make_halomap_dict(self["snapshot_final/halo_map"][:])
             elif key == "npart":
                 halomap = self["particle_offset"]
                 out = numpy.zeros(len(halomap), dtype=numpy.int32)
                 for i, hid in enumerate(self["index"]):
-                    if i == 0:
+                    if hid == 0:
                         continue
                     start, end = halomap[hid]
                     out[i] = end - start
+            elif key in self.data[self.catalogue_name].keys():
+                out = self.data[f"{self.catalogue_name}/{key}"][:]
             else:
                 raise KeyError(f"Key '{key}' is not available.")
 
-        # TODO enforce a maximum size of the dictionary
-        # ALSO DO THE MASKING HERE? AND IF A NEW MASK THEN RESET CACHE?
+        # TODO: Enfore the masking somewhere here?
+        if self.cache_length() > self.cache_maxsize:
+            self._cache.popitem(last=False)
         self._cache[key] = out
         return out
 
@@ -496,7 +517,12 @@ class BaseCatalogue(ABC):
         if not self._is_closed:
             self.data.close()
             self._is_closed = True
-        self._cache = {}
+        self._cache.clear()
+        collect()
+
+    def clear_cache(self):
+        """Clear the cache dictionary."""
+        self._cache.clear()
         collect()
 
     def __len__(self):
@@ -533,10 +559,11 @@ class CSiBORGCatalogue(BaseCatalogue):
         Observer's velocity in :math:`\mathrm{km} / \mathrm{s}`.
     """
     def __init__(self, nsim, paths, catalogue_name, halo_finder, mass_key=None,
-                 observer_velocity=None):
+                 observer_velocity=None, cache_maxsize=64):
         super().__init__("csiborg", nsim,
                          max(paths.get_snapshots(nsim, "csiborg")),
-                         halo_finder, catalogue_name, paths, mass_key)
+                         halo_finder, catalogue_name, paths, mass_key,
+                         cache_maxsize)
         self.box = CSiBORGBox(self.nsnap, self.nsim, self.paths)
         self.observer_location = [338.85, 338.85, 338.85]  # Mpc / h
         self.observer_velocity = observer_velocity