diff --git a/validator_client/src/duties_service.rs b/validator_client/src/duties_service.rs
index 3f9912cfd1..e236953fd3 100644
--- a/validator_client/src/duties_service.rs
+++ b/validator_client/src/duties_service.rs
@@ -15,6 +15,9 @@ use types::{ChainSpec, Epoch, EthSpec, PublicKey, Slot};
 /// Delay this period of time after the slot starts. This allows the node to process the new slot.
 const TIME_DELAY_FROM_SLOT: Duration = Duration::from_millis(100);
 
+/// Remove any duties where the `duties_epoch < current_epoch - PRUNE_DEPTH`.
+const PRUNE_DEPTH: u64 = 4;
+
 type BaseHashMap = HashMap<PublicKey, HashMap<Epoch, ValidatorDuty>>;
 
 enum InsertOutcome {
@@ -108,7 +111,6 @@ impl DutiesStore {
         }
     }
 
-    // TODO: call this.
     fn prune(&self, prior_to: Epoch) {
         self.store
             .write()
@@ -250,21 +252,39 @@ impl<T: SlotClock + Clone + 'static, E: EthSpec> DutiesService<T, E> {
     }
 
     fn do_update(self) -> impl Future<Item = (), Error = ()> {
-        let slots_per_epoch = E::slots_per_epoch();
         let service_1 = self.clone();
         let service_2 = self.clone();
-        let log = self.context.log.clone();
+        let service_3 = self.clone();
+        let log_1 = self.context.log.clone();
+        let log_2 = self.context.log.clone();
 
         self.slot_clock
             .now()
             .ok_or_else(move || {
-                error!(log, "Duties manager failed to read slot clock");
+                error!(log_1, "Duties manager failed to read slot clock");
             })
             .into_future()
-            .map(move |slot| slot.epoch(slots_per_epoch))
+            .map(move |slot| {
+                let epoch = slot.epoch(E::slots_per_epoch());
+
+                if slot % E::slots_per_epoch() == 0 {
+                    let prune_below = epoch - PRUNE_DEPTH;
+
+                    trace!(
+                        log_2,
+                        "Pruning duties cache";
+                        "pruning_below" => prune_below.as_u64(),
+                        "current_epoch" => epoch.as_u64(),
+                    );
+
+                    service_1.store.prune(prune_below);
+                }
+
+                epoch
+            })
             .and_then(move |epoch| {
-                let log = service_1.context.log.clone();
-                service_1.update_epoch(epoch).then(move |result| {
+                let log = service_2.context.log.clone();
+                service_2.update_epoch(epoch).then(move |result| {
                     if let Err(e) = result {
                         error!(
                             log,
@@ -273,8 +293,8 @@ impl<T: SlotClock + Clone + 'static, E: EthSpec> DutiesService<T, E> {
                         );
                     }
 
-                    let log = service_2.context.log.clone();
-                    service_2.update_epoch(epoch + 1).map_err(move |e| {
+                    let log = service_3.context.log.clone();
+                    service_3.update_epoch(epoch + 1).map_err(move |e| {
                         error!(
                             log,
                             "Failed to get next epoch duties";