diff --git a/rest_framework/serializers.py b/rest_framework/serializers.py
index 53dcec163..4519ab053 100644
--- a/rest_framework/serializers.py
+++ b/rest_framework/serializers.py
@@ -60,7 +60,7 @@ def _get_declared_fields(bases, attrs):
 
     # If this class is subclassing another Serializer, add that Serializer's
     # fields.  Note that we loop over the bases in *reverse*. This is necessary
-    # in order to the correct order of fields.
+    # in order to maintain the correct order of fields.
     for base in bases[::-1]:
         if hasattr(base, 'base_fields'):
             fields = base.base_fields.items() + fields
@@ -94,7 +94,6 @@ class BaseSerializer(Field):
     def __init__(self, instance=None, data=None, files=None, context=None, partial=False, **kwargs):
         super(BaseSerializer, self).__init__(**kwargs)
         self.opts = self._options_class(self.Meta)
-        self.fields = copy.deepcopy(self.base_fields)
         self.parent = None
         self.root = None
         self.partial = partial
@@ -104,7 +103,7 @@ class BaseSerializer(Field):
         self.init_data = data
         self.init_files = files
         self.object = instance
-        self.default_fields = self.get_default_fields()
+        self.fields = self.get_fields()
 
         self._data = None
         self._files = None
@@ -129,13 +128,15 @@ class BaseSerializer(Field):
         ret = SortedDict()
 
         # Get the explicitly declared fields
-        for key, field in self.fields.items():
+        base_fields = copy.deepcopy(self.base_fields)
+        for key, field in base_fields.items():
             ret[key] = field
             # Set up the field
             field.initialize(parent=self, field_name=key)
 
         # Add in the default fields
-        for key, val in self.default_fields.items():
+        default_fields = self.get_default_fields()
+        for key, val in default_fields.items():
             if key not in ret:
                 ret[key] = val
 
@@ -182,8 +183,7 @@ class BaseSerializer(Field):
         ret = self._dict_class()
         ret.fields = {}
 
-        fields = self.get_fields()
-        for field_name, field in fields.items():
+        for field_name, field in self.fields.items():
             key = self.get_field_key(field_name)
             value = field.field_to_native(obj, field_name)
             ret[key] = value
@@ -195,9 +195,8 @@ class BaseSerializer(Field):
         Core of deserialization, together with `restore_object`.
         Converts a dictionary of data into a dictionary of deserialized fields.
         """
-        fields = self.get_fields()
         reverted_data = {}
-        for field_name, field in fields.items():
+        for field_name, field in self.fields.items():
             try:
                 field.field_from_native(data, files, field_name, reverted_data)
             except ValidationError as err:
@@ -209,10 +208,7 @@ class BaseSerializer(Field):
         """
         Run `validate_<fieldname>()` and `validate()` methods on the serializer
         """
-        # TODO: refactor this so we're not determining the fields again
-        fields = self.get_fields()
-
-        for field_name, field in fields.items():
+        for field_name, field in self.fields.items():
             try:
                 validate_method = getattr(self, 'validate_%s' % field_name, None)
                 if validate_method: